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Chapter 5. MySQL Server AdministrationEnd of Product LifecycleActive development and support for MySQL Database Server version 5.0 has ended. However, there is still extended support available. For details, see http://www.mysql.com/about/legal/lifecycle/#calendar. According to the MySQL Lifecycle Policy (see http://www.mysql.com/about/legal/lifecycle/#policy), only Security and Severity Level 1 issues will still be fixed for MySQL 5.0. Please consider upgrading to a recent version.
MySQL Server (mysqld) is the main program that
does most of the work in a MySQL installation. This section provides
an overview of MySQL Server and covers topics that deal with
administering a MySQL installation:
mysqld is the MySQL server. The following
discussion covers these MySQL server configuration topics:
Startup options that the server supports
Server system variables
Server status variables
How to set the server SQL mode
The server shutdown process
Note
Not all storage engines are supported by all MySQL server binaries
and configurations. To find out how to determine which storage
engines your MySQL server installation supports, see
Section 12.5.5.13, “SHOW ENGINES Syntax”.
5.1.1. Server Option and Variable Reference
The following table provides a list of all the command line
options, server and status variables applicable within
mysqld.
The table lists command-line options (Cmd-line), options valid in
configuration files (Option file), server system variables (System
Var), and status variables (Status var) in one unified list, with
notification of where each option/variable is valid. If a server
option set on the command line or in an option file differs from
the name of the corresponding server system or status variable,
the variable name is noted immediately below the corresponding
option. For status variables, the scope of the variable is shown
(Scope) as either global, session, or both. Please see the
corresponding sections for details on setting and using the
options and variables. Where appropriate, a direct link to further
information on the item as available.
Table 5.1. Option/Variable Summary 5.1.2. Server Command Options
When you start the mysqld server, you can
specify program options using any of the methods described in
Section 4.2.3, “Specifying Program Options”. The most common methods are to
provide options in an option file or on the command line. However,
in most cases it is desirable to make sure that the server uses
the same options each time it runs. The best way to ensure this is
to list them in an option file. See
Section 4.2.3.3, “Using Option Files”.
MySQL Enterprise
For expert advice on setting command options, subscribe to the
MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
mysqld reads options from the
[mysqld] and [server]
groups. mysqld_safe reads options from the
[mysqld], [server],
[mysqld_safe], and
[safe_mysqld] groups.
mysql.server reads options from the
[mysqld] and [mysql.server]
groups.
An embedded MySQL server usually reads options from the
[server], [embedded], and
[xxxxx_SERVER]
groups, where xxxxx is the name of the
application into which the server is embedded.
mysqld accepts many command options. For a
brief summary, execute mysqld --help. To see
the full list, use mysqld --verbose --help.
The following list shows some of the most common server options.
Additional options are described in other sections:
Options that affect security: See
Section 5.3.3, “Security-Related mysqld Options”.
SSL-related options: See Section 5.5.7.3, “SSL Command Options”.
Binary log control options: See
Section 16.1.2.4, “Binary Log Options and Variables”.
Replication-related options: See
Section 16.1.2, “Replication and Binary Logging Options and Variables”.
Options specific to particular storage engines: See
Section 13.1.1, “MyISAM Startup Options”, Section 13.5.3, “BDB Startup Options”,
Section 13.2.3, “InnoDB Startup Options and System Variables”, and
Section 17.3.4.2, “mysqld Command Options for MySQL Cluster”.
You can also set the values of server system variables by using
variable names as options, as described at the end of this
section.
Some options control the size of buffers or caches. For a given
buffer, the server might need to allocate internal data
structures. These structures typically are allocated from the
total memory allocated to the buffer, and the amount of space
required might be platform dependent. This means that when you
assign a value to an option that controls a buffer size, the
amount of space actually available might differ from the value
assigned. In some cases, the amount might be less than the value
assigned. It is also possible that the server will adjust a value
upward. For example, if you assign a value of 0 to an option for
which the minimal value is 1024, the server will set the value to
1024.
Values for buffer sizes, lengths, and stack sizes are given in
bytes unless otherwise specified.
Some options take file name values. Unless otherwise specified,
the default file location is the data directory if the value is a
relative path name. To specify the location explicitly, use an
absolute path name. Suppose that the data directory is
/var/mysql/data. If a file-valued option is
given as a relative path name, it will be located under
/var/mysql/data. If the value is an absolute
path name, its location is as given by the path name.
--help, -?
Display a short help message and exit. Use both the
--verbose and
--help options to see the full
message.
--allow-suspicious-udfs
This option controls whether user-defined functions that have
only an xxx symbol for the main function
can be loaded. By default, the option is off and only UDFs
that have at least one auxiliary symbol can be loaded; this
prevents attempts at loading functions from shared object
files other than those containing legitimate UDFs. This option
was added in version 5.0.3. See
Section 21.2.2.6, “User-Defined Function Security Precautions”.
--ansi
Use standard (ANSI) SQL syntax instead of MySQL syntax. For
more precise control over the server SQL mode, use the
--sql-mode option instead. See
Section 1.8.3, “Running MySQL in ANSI Mode”, and
Section 5.1.7, “Server SQL Modes”.
--basedir=path,
-b
path
The path to the MySQL installation directory. All paths are
usually resolved relative to this directory.
--big-tables
Allow large result sets by saving all temporary sets in files.
This option prevents most “table full” errors,
but also slows down queries for which in-memory tables would
suffice. Since MySQL 3.23.2, the server is able to handle
large result sets automatically by using memory for small
temporary tables and switching to disk tables where necessary.
--bind-address=IP
The IP address to bind to. Only one address can be selected.
If this option is specified multiple times, the last address
given is used.
If no address or 0.0.0.0 is specified, the
server listens on all interfaces.
--bootstrap
This option is used by the mysql_install_db
script to create the MySQL privilege tables without having to
start a full MySQL server.
This option is unavailable if MySQL was configured with the
--disable-grant-options
option. See Section 2.16.2, “Typical configure Options”.
--character-sets-dir=path
The directory where character sets are installed. See
Section 9.2, “The Character Set Used for Data and Sorting”.
--character-set-client-handshake
Don't ignore character set information sent by the client. To
ignore client information and use the default server character
set, use
--skip-character-set-client-handshake;
this makes MySQL behave like MySQL 4.0.
--character-set-filesystem=charset_name
The file system character set. This option sets the
character_set_filesystem
system variable. It was added in MySQL 5.0.19.
--character-set-server=charset_name,
-C charset_name
Use charset_name as the default
server character set. See
Section 9.2, “The Character Set Used for Data and Sorting”. If you use this
option to specify a nondefault character set, you should also
use --collation-server to
specify the collation.
--chroot=path,
-r path
Put the mysqld server in a closed
environment during startup by using the
chroot() system call. This is a recommended
security measure. Note that use of this option somewhat limits
LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE.
--collation-server=collation_name
Use collation_name as the default
server collation. See Section 9.2, “The Character Set Used for Data and Sorting”.
--console
(Windows only.) Write error log messages to
stderr and stdout even
if --log-error is specified.
mysqld does not close the console window if
this option is used.
--core-file
Write a core file if mysqld dies. The name
and location of the core file is system dependent. On Linux, a
core file named
core.pid is
written to the current working directory of the process, which
for mysqld is the data directory.
pid represents the process ID of
the server process. On Mac OS X, a core file named
core.pid is
written to the /cores directory. On
Solaris, use the coreadm command to specify
where to write the core file and how to name it.
For some systems, to get a core file you must also specify the
--core-file-size option to
mysqld_safe. See
Section 4.3.2, “mysqld_safe — MySQL Server Startup Script”. On some systems, such as
Solaris, you do not get a core file if you are also using the
--user option. There might be
additional restrictions or limitations. For example, it might
be necessary to execute ulimit -c unlimited
before starting the server. Consult your system documentation.
--datadir=path,
-h path
The path to the data directory.
--debug[=debug_options],
-# [debug_options]
If MySQL is configured with
--with-debug, you can use
this option to get a trace file of what
mysqld is doing. A typical
debug_options string is
'd:t:o,file_name'.
The default is 'd:t:i:o,mysqld.trace'. See
MySQL
Internals: Porting.
As of MySQL 5.0.25, using
--with-debug to configure
MySQL with debugging support enables you to use the
--debug="d,parser_debug" option
when you start the server. This causes the Bison parser that
is used to process SQL statements to dump a parser trace to
the server's standard error output. Typically, this output is
written to the error log.
--default-character-set=charset_name
(DEPRECATED)
Use charset_name as the default
character set. This option is deprecated in favor of
--character-set-server. See
Section 9.2, “The Character Set Used for Data and Sorting”.
--default-collation=collation_name
Use collation_name as the default
collation. This option is deprecated in favor of
--collation-server. See
Section 9.2, “The Character Set Used for Data and Sorting”.
--default-storage-engine=type
Set the default storage engine (table type) for tables. See
Chapter 13, Storage Engines.
--default-table-type=type
This option is a deprecated synonym for
--default-storage-engine.
--default-time-zone=timezone
Set the default server time zone. This option sets the global
time_zone system variable. If
this option is not given, the default time zone is the same as
the system time zone (given by the value of the
system_time_zone system
variable.
--delay-key-write[={OFF|ON|ALL}]
Specify how to use delayed key writes. Delayed key writing
causes key buffers not to be flushed between writes for
MyISAM tables. OFF
disables delayed key writes. ON enables
delayed key writes for those tables that were created with the
DELAY_KEY_WRITE option.
ALL delays key writes for all
MyISAM tables. See
Section 7.5.3, “Tuning Server Parameters”, and
Section 13.1.1, “MyISAM Startup Options”.
Note
If you set this variable to ALL, you
should not use MyISAM tables from within
another program (such as another MySQL server or
myisamchk) when the tables are in use.
Doing so leads to index corruption.
--des-key-file=file_name
Read the default DES keys from this file. These keys are used
by the DES_ENCRYPT() and
DES_DECRYPT() functions.
--enable-named-pipe
Enable support for named pipes. This option can be used only
with the mysqld-nt and
mysqld-debug servers that support
named-pipe connections.
--enable-pstack
Print a symbolic stack trace on failure. This capability is
available only on Intel Linux systems, and only if MySQL was
configured with the --with-pstack option.
--engine-condition-pushdown={ON|OFF}
Sets the
engine_condition_pushdown
system variable. For more information, see
Section 7.2.7, “Condition Pushdown Optimization”.
This variable was added in MySQL 5.0.3.
--exit-info[=flags],
-T [flags]
This is a bit mask of different flags that you can use for
debugging the mysqld server. Do not use
this option unless you know exactly what
it does!
--external-locking
Enable external locking (system locking), which is disabled by
default as of MySQL 4.0. Note that if you use this option on a
system on which lockd does not fully work
(such as Linux), it is easy for mysqld to
deadlock. This option previously was named
--enable-locking.
For more information about external locking, including
conditions under which it can and cannot be used, see
Section 7.3.4, “External Locking”.
--flush
Flush (synchronize) all changes to disk after each SQL
statement. Normally, MySQL does a write of all changes to disk
only after each SQL statement and lets the operating system
handle the synchronizing to disk. See
Section B.5.4.2, “What to Do If MySQL Keeps Crashing”.
--gdb
Install an interrupt handler for SIGINT
(needed to stop mysqld with
^C to set breakpoints) and disable stack
tracing and core file handling. See
MySQL
Internals: Porting.
--init-file=file_name
Read SQL statements from this file at startup. Each statement
must be on a single line and should not include comments.
This option is unavailable if MySQL was configured with the
--disable-grant-options
option. See Section 2.16.2, “Typical configure Options”.
--innodb_safe_binlog
If this option is given, then after a crash recovery by
InnoDB, mysqld truncates
the binary log after the last not-rolled-back transaction in
the log. The option also causes InnoDB to
print an error if the binary log is smaller or shorter than it
should be. See Section 5.2.3, “The Binary Log”. This option was
removed in MySQL 5.0.3, having been made obsolete by the
introduction of XA transaction support.
--innodb-xxx
The InnoDB options are listed in
Section 13.2.3, “InnoDB Startup Options and System Variables”.
--language=lang_name,
-L lang_name
The language to use for error messages.
lang_name can be given as the
language name or as the full path name to the directory where
the language files are installed. See
Section 9.3, “Setting the Error Message Language”.
--large-pages
Some hardware/operating system architectures support memory
pages greater than the default (usually 4KB). The actual
implementation of this support depends on the underlying
hardware and operating system. Applications that perform a lot
of memory accesses may obtain performance improvements by
using large pages due to reduced Translation Lookaside Buffer
(TLB) misses.
Currently, MySQL supports only the Linux implementation of
large page support (which is called HugeTLB in Linux). See
Section 7.5.9, “Enabling Large Page Support”.
--large-pages is disabled by
default. It was added in MySQL 5.0.3.
--log[=file_name],
-l [file_name]
Log connections and SQL statements received from clients to
this file. See Section 5.2.2, “The General Query Log”. If you omit the
file name, MySQL uses
host_name.log
--log-error[=file_name]
Log errors and startup messages to this file. See
Section 5.2.1, “The Error Log”. If you omit the file name, MySQL
uses
host_name.err.err.
--log-isam[=file_name]
Log all MyISAM changes to this file (used
only when debugging MyISAM).
--log-long-format
(DEPRECATED)
Log extra information to the update log, binary update log,
and slow query log, if they have been activated. For example,
the user name and timestamp are logged for all queries. This
option is deprecated, as it now represents the default logging
behavior. (See the description for
--log-short-format.) The
--log-queries-not-using-indexes
option is available for the purpose of logging queries that do
not use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with the slow query log enabled,
queries that are expected to retrieve all rows are logged. See
Section 5.2.4, “The Slow Query Log”. This option does not
necessarily mean that no index is used. For example, a query
that uses a full index scan uses an index but would be logged
because the index would not limit the number of rows.
--log-short-format
Originally intended to log less information to the update log,
binary log and slow query log, if they have been activated.
However, this option is not operational.
--log-slow-admin-statements
Log slow administrative statements such as
OPTIMIZE TABLE,
ANALYZE TABLE, and
ALTER TABLE to the slow query
log.
--log-slow-queries[=file_name]
Log all queries that have taken more than
long_query_time seconds to
execute to this file. See Section 5.2.4, “The Slow Query Log”.
See the descriptions of the
--log-long-format and
--log-short-format options for
details.
--log-tc=file_name
The name of the memory-mapped transaction coordinator log file
(for XA transactions that affect multiple storage engines when
the binary log is disabled). The default name is
tc.log. The file is created under the
data directory if not given as a full path name. Currently,
this option is unused. Added in MySQL 5.0.3.
--log-tc-size=size
The size in bytes of the memory-mapped transaction coordinator
log. The default size is 24KB. Added in MySQL 5.0.3.
--log-warnings[=level],
-W [level]
Print out warnings such as Aborted
connection... to the error log. Enabling this option
is recommended, for example, if you use replication (you get
more information about what is happening, such as messages
about network failures and reconnections). This option is
enabled (1) by default, and the default
level value if omitted is 1. To
disable this option, use
--log-warnings=0. If the value
is greater than 1, aborted connections are written to the
error log. See Section B.5.2.11, “Communication Errors and Aborted Connections”.
If a slave server was started with
--log-warnings enabled, the
slave prints messages to the error log to provide information
about its status, such as the binary log and relay log
coordinates where it starts its job, when it is switching to
another relay log, when it reconnects after a disconnect, and
so forth.
--low-priority-updates
Give table-modifying operations
(INSERT,
REPLACE,
DELETE,
UPDATE) lower priority than
selects. This can also be done via {INSERT | REPLACE
| DELETE | UPDATE} LOW_PRIORITY ... to lower the
priority of only one query, or by SET
LOW_PRIORITY_UPDATES=1 to change the priority in one
thread. This affects only storage engines that use only
table-level locking (MyISAM,
MEMORY, MERGE). See
Section 7.3.2, “Table Locking Issues”.
--memlock
Lock the mysqld process in memory. This
option might help if you have a problem where the operating
system is causing mysqld to swap to disk.
--memlock works on systems that
support the mlockall() system call; this
includes Solaris as well as most Linux distributions that use
a 2.4 or newer kernel. On Linux systems, you can tell whether
or not mlockall() (and thus this option) is
supported by checking to see whether or not it is defined in
the system mman.h file, like this:
shell> grep mlockall /usr/include/sys/mman.h
If mlockall() is supported, you should see
in the output of the previous command something like the
following:
extern int mlockall (int __flags) __THROW;
Important
Using this option requires that you run the server as
root, which, for reasons of security, is
normally not a good idea. See
Section 5.3.5, “How to Run MySQL as a Normal User”.
You must not try to use this option on a system that does
not support the mlockall() system call;
if you do so, mysqld will very likely
crash as soon as you try to start it.
--myisam-block-size=N
The block size to be used for MyISAM index
pages.
--myisam-recover[=option[,option]...]]
Set the MyISAM storage engine recovery
mode. The option value is any combination of the values of
DEFAULT, BACKUP,
FORCE, or QUICK. If you
specify multiple values, separate them by commas. Specifying
the option with no argument is the same as specifying
DEFAULT, and specifying with an explicit
value of "" disables recovery (same as not
giving the option). If recovery is enabled, each time
mysqld opens a MyISAM
table, it checks whether the table is marked as crashed or
wasn't closed properly. (The last option works only if you are
running with external locking disabled.) If this is the case,
mysqld runs a check on the table. If the
table was corrupted, mysqld attempts to
repair it.
The following options affect how the repair works.
Before the server automatically repairs a table, it writes a
note about the repair to the error log. If you want to be able
to recover from most problems without user intervention, you
should use the options BACKUP,FORCE. This
forces a repair of a table even if some rows would be deleted,
but it keeps the old data file as a backup so that you can
later examine what happened.
See Section 13.1.1, “MyISAM Startup Options”.
--old-passwords
Force the server to generate short (pre-4.1) password hashes
for new passwords. This is useful for compatibility when the
server must support older client programs. See
Section 5.5.6.3, “Password Hashing in MySQL”.
--old-style-user-limits
Enable old-style user limits. (Before MySQL 5.0.3, account
resource limits were counted separately for each host from
which a user connected rather than per account row in the
user table.) See
Section 5.5.4, “Limiting Account Resources”. This option was added in
MySQL 5.0.3.
--one-thread
Only use one thread (for debugging under Linux). This option
is available only if the server is built with debugging
enabled. See
MySQL
Internals: Porting.
--open-files-limit=count
Changes the number of file descriptors available to
mysqld. You should try increasing the value
of this option if mysqld gives you the
error Too many open files.
mysqld uses the option value to reserve
descriptors with setrlimit(). If the
requested number of file descriptors cannot be allocated,
mysqld writes a warning to the error log.
mysqld may attempt to allocate more than
the requested number of descriptors (if they are available),
using the values of
max_connections and
table_cache to estimate
whether more descriptors will be needed.
--pid-file=path
The path name of the process ID file. The server creates the
file in the data directory unless an absolute path name is
given to specify a different directory. This file is used by
other programs such as mysqld_safe to
determine the server's process ID.
--port=port_num,
-P port_num
The port number to use when listening for TCP/IP connections.
The port number must be 1024 or higher unless the server is
started by the root system user.
--port-open-timeout=num
On some systems, when the server is stopped, the TCP/IP port
might not become available immediately. If the server is
restarted quickly afterward, its attempt to reopen the port
can fail. This option indicates how many seconds the server
should wait for the TCP/IP port to become free if it cannot be
opened. The default is not to wait. This option was added in
MySQL 5.0.19.
--safe-mode
Skip some optimization stages.
--safe-show-database
(DEPRECATED)
See Section 5.4.1, “Privileges Provided by MySQL”.
--safe-user-create
If this option is enabled, a user cannot create new MySQL
users by using the GRANT
statement unless the user has the
INSERT privilege for the
mysql.user table or any column in the
table. If you want a user to have the ability to create new
users that have those privileges that the user has the right
to grant, you should grant the user the following privilege:
GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This ensures that the user cannot change any privilege columns
directly, but has to use the
GRANT statement to give
privileges to other users.
--secure-auth
Disallow authentication by clients that attempt to use
accounts that have old (pre-4.1) passwords.
--secure-file-priv=path
This option limits the effect of the
LOAD_FILE() function and the
LOAD DATA and
SELECT ... INTO
OUTFILE statements to work only with files in the
specified directory.
This option was added in MySQL 5.0.38.
--shared-memory
Enable shared-memory connections by local clients. This option
is available only on Windows.
--shared-memory-base-name=name
The name of shared memory to use for shared-memory
connections. This option is available only on Windows. The
default name is MYSQL. The name is case
sensitive.
--skip-bdb
Disable the BDB storage engine. This saves
memory and might speed up some operations. Do not use this
option if you require BDB tables.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on
MyISAM tables. (This is to be used only if
you think you have found a bug in this feature.) See
Section 7.3.3, “Concurrent Inserts”.
--skip-external-locking
Do not use external locking (system locking). For more
information about external locking, including conditions under
which it can and cannot be used, see
Section 7.3.4, “External Locking”.
External locking has been disabled by default since MySQL 4.0.
--skip-grant-tables
This option causes the server to start without using the
privilege system at all, which gives anyone with access to the
server unrestricted access to all
databases. You can cause a running server to start
using the grant tables again by executing mysqladmin
flush-privileges or mysqladmin
reload command from a system shell, or by issuing a
MySQL FLUSH
PRIVILEGES statement after connecting to the server.
This option also suppresses loading of user-defined functions
(UDFs).
This option is unavailable if MySQL was configured with the
--disable-grant-options
option. See Section 2.16.2, “Typical configure Options”.
--skip-host-cache
Do not use the internal host name cache for faster name-to-IP
resolution. Instead, query the DNS server every time a client
connects. See Section 7.5.11, “How MySQL Uses DNS”.
--skip-innodb
Disable the InnoDB storage engine. This
saves memory and disk space and might speed up some
operations. Do not use this option if you require
InnoDB tables.
--skip-merge
Disable the MERGE storage engine. This
option was added in MySQL 5.0.24. It can be used if the
following behavior is undesirable: If a user has access to
MyISAM table t,
that user can create a MERGE table
m that accesses
t. However, if the user's
privileges on t are subsequently
revoked, the user can continue to access
t by doing so through
m.
--skip-name-resolve
Do not resolve host names when checking client connections.
Use only IP numbers. If you use this option, all
Host column values in the grant tables must
be IP numbers or localhost. See
Section 7.5.11, “How MySQL Uses DNS”.
--skip-networking
Don't listen for TCP/IP connections at all. All interaction
with mysqld must be made via named pipes or
shared memory (on Windows) or Unix socket files (on Unix).
This option is highly recommended for systems where only local
clients are allowed. See Section 7.5.11, “How MySQL Uses DNS”.
--ssl*
Options that begin with --ssl
specify whether to allow clients to connect via SSL and
indicate where to find SSL keys and certificates. See
Section 5.5.7.3, “SSL Command Options”.
--standalone
Instructs the MySQL server not to run as a service.
--symbolic-links,
--skip-symbolic-links
Enable or disable symbolic link support. This option has
different effects on Windows and Unix:
--skip-safemalloc
If MySQL is configured with
--with-debug=full, all MySQL
programs check for memory overruns during each memory
allocation and memory freeing operation. This checking is very
slow, so for the server you can avoid it when you don't need
it by using the
--skip-safemalloc option.
--skip-show-database
With this option, the SHOW
DATABASES statement is allowed only to users who
have the SHOW DATABASES
privilege, and the statement displays all database names.
Without this option, SHOW
DATABASES is allowed to all users, but displays each
database name only if the user has the
SHOW DATABASES privilege or
some privilege for the database. Note that
any global privilege is considered a
privilege for the database.
--skip-stack-trace
Don't write stack traces. This option is useful when you are
running mysqld under a debugger. On some
systems, you also must use this option to get a core file. See
MySQL
Internals: Porting.
--skip-thread-priority
Disable using thread priorities for faster response time.
mysqld makes a large number of invalid
calls to thread scheduling routines on Linux. These calls do
not affect performance noticeably but may be a source of
“noise” for debugging tools. For example, they
can overwhelm other information of more interest in kernel
logs. To avoid these calls, start the server with the
--skip-thread-priority option.
--socket=path
On Unix, this option specifies the Unix socket file to use
when listening for local connections. The default value is
/tmp/mysql.sock. If this option is given,
the server creates the file in the data directory unless an
absolute path name is given to specify a different directory.
On Windows, the option specifies the pipe name to use when
listening for local connections that use a named pipe. The
default value is MySQL (not case
sensitive).
--sql-mode=value[,value[,value...]]
Set the SQL mode. See Section 5.1.7, “Server SQL Modes”.
--sysdate-is-now
As of MySQL 5.0.12, SYSDATE()
by default returns the time at which it executes, not the time
at which the statement in which it occurs begins executing.
This differs from the behavior of
NOW(). This option causes
SYSDATE() to be an alias for
NOW(). For information about
the implications for binary logging and replication, see the
description for SYSDATE() in
Section 11.6, “Date and Time Functions” and for SET
TIMESTAMP in
Section 5.1.4, “Session System Variables”.
This option was added in MySQL 5.0.20.
--tc-heuristic-recover={COMMIT|ROLLBACK}
The type of decision to use in the heuristic recovery process.
Currently, this option is unused. Added in MySQL 5.0.3.
--temp-pool
This option causes most temporary files created by the server
to use a small set of names, rather than a unique name for
each new file. This works around a problem in the Linux kernel
dealing with creating many new files with different names.
With the old behavior, Linux seems to “leak”
memory, because it is being allocated to the directory entry
cache rather than to the disk cache.
--transaction-isolation=level
Sets the default transaction isolation level. The
level value can be
READ-UNCOMMITTED,
READ-COMMITTED,
REPEATABLE-READ, or
SERIALIZABLE. See
Section 12.4.6, “SET TRANSACTION Syntax”.
--tmpdir=path,
-t path
The path of the directory to use for creating temporary files.
It might be useful if your default /tmp
directory resides on a partition that is too small to hold
temporary tables. This option accepts several paths that are
used in round-robin fashion. Paths should be separated by
colon characters (“:”) on Unix
and semicolon characters (“;”)
on Windows, NetWare, and OS/2. If the MySQL server is acting
as a replication slave, you should not set
--tmpdir to point to a
directory on a memory-based file system or to a directory that
is cleared when the server host restarts. For more information
about the storage location of temporary files, see
Section B.5.4.4, “Where MySQL Stores Temporary Files”. A replication slave needs
some of its temporary files to survive a machine restart so
that it can replicate temporary tables or
LOAD DATA
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
fails.
--user={user_name|user_id},
-u
{user_name|user_id}
Run the mysqld server as the user having
the name user_name or the numeric
user ID user_id.
(“User” in this context refers to a system login
account, not a MySQL user listed in the grant tables.)
This option is mandatory when starting
mysqld as root. The
server changes its user ID during its startup sequence,
causing it to run as that particular user rather than as
root. See
Section 5.3.1, “General Security Guidelines”.
To avoid a possible security hole where a user adds a
--user=root option to a
my.cnf file (thus causing the server to
run as root), mysqld
uses only the first --user
option specified and produces a warning if there are multiple
--user options. Options in
/etc/my.cnf and
$MYSQL_HOME/my.cnf are processed before
command-line options, so it is recommended that you put a
--user option in
/etc/my.cnf and specify a value other
than root. The option in
/etc/my.cnf is found before any other
--user options, which ensures
that the server runs as a user other than
root, and that a warning results if any
other --user option is found.
--verbose,
-v
Use this option with the --help
option for detailed help.
--version, -V
Display version information and exit.
You can assign a value to a server system variable by using an
option of the form
--var_name=value.
For example, --key_buffer_size=32M
sets the key_buffer_size variable
to a value of 32MB.
Note that when you assign a value to a variable, MySQL might
automatically correct the value to stay within a given range, or
adjust the value to the closest allowable value if only certain
values are allowed.
If you want to restrict the maximum value to which a variable can
be set at runtime with
SET, you can
define this by using the
--maximum-var_name=value
command-line option.
It is also possible to set variables by using
--set-variable=var_name=value
or -O
var_name=value
syntax. This syntax is deprecated.
You can change the values of most system variables for a running
server with the
SET
statement. See Section 12.5.4, “SET Syntax”.
Section 5.1.3, “Server System Variables”, provides a full
description for all variables, and additional information for
setting them at server startup and runtime.
Section 7.5.3, “Tuning Server Parameters”, includes information on
optimizing the server by tuning system variables.
5.1.3. Server System Variables
The MySQL server maintains many system variables that indicate how
it is configured. Each system variable has a default value. System
variables can be set at server startup using options on the
command line or in an option file. Most of them can be changed
dynamically while the server is running by means of the
SET
statement, which enables you to modify operation of the server
without having to stop and restart it. You can refer to system
variable values in expressions.
There are several ways to see the names and values of system
variables:
To see the values that a server will use based on its
compiled-in defaults and any option files that it reads, use
this command:
mysqld --verbose --help
To see the values that a server will use based on its
compiled-in defaults, ignoring the settings in any option
files, use this command:
mysqld --no-defaults --verbose --help
To see the current values used by a running server, use the
SHOW VARIABLES statement.
This section provides a description of each system variable.
Variables with no version indicated are present in all MySQL
5.0 releases. For historical information concerning
their implementation, please see
http://www.mysql.com/products/enterprise//4.1/en/.
The following table lists all available system variables:
Table 5.2. System Variable Summary
For additional system variable information, see these sections:
Section 5.1.4, “Session System Variables”, describes system
variables that exist only as session variables (that is, they
do not have any global counterpart).
Section 5.1.5, “Using System Variables”, discusses the syntax
for setting and displaying system variable values.
Section 5.1.5.2, “Dynamic System Variables”, lists the
variables that can be set at runtime.
Information on tuning system variables can be found in
Section 7.5.3, “Tuning Server Parameters”.
Section 13.2.3, “InnoDB Startup Options and System Variables”, lists
InnoDB system variables.
Section 17.3.4.3, “MySQL Cluster System Variables”, lists system
variables which are specific to MySQL Cluster.
For information on server system variables specific to
replication, see Section 16.1.2, “Replication and Binary Logging Options and Variables”.
Note
Some of the following variable descriptions refer to
“enabling” or “disabling” a variable.
These variables can be enabled with the
SET
statement by setting them to ON or
1, or disabled by setting them to
OFF or 0. However, to set
such a variable on the command line or in an option file, you
must set it to 1 or 0;
setting it to ON or OFF
will not work. For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
Some system variables control the size of buffers or caches. For a
given buffer, the server might need to allocate internal data
structures. These structures typically are allocated from the
total memory allocated to the buffer, and the amount of space
required might be platform dependent. This means that when you
assign a value to a system variable that controls a buffer size,
the amount of space actually available might differ from the value
assigned. In some cases, the amount might be less than the value
assigned. It is also possible that the server will adjust a value
upward. For example, if you assign a value of 0 to a variable for
which the minimal value is 1024, the server will set the value to
1024.
Values for buffer sizes, lengths, and stack sizes are given in
bytes unless otherwise specified.
Some system variables take file name values. Unless otherwise
specified, the default file location is the data directory if the
value is a relative path name. To specify the location explicitly,
use an absolute path name. Suppose that the data directory is
/var/mysql/data. If a file-valued variable is
given as a relative path name, it will be located under
/var/mysql/data. If the value is an absolute
path name, its location is as given by the path name.
automatic_sp_privileges
When this variable has a value of 1 (the default), the server
automatically grants the
EXECUTE and
ALTER ROUTINE privileges to the
creator of a stored routine, if the user cannot already
execute and alter or drop the routine. (The
ALTER ROUTINE privilege is
required to drop the routine.) The server also automatically
drops those privileges from the creator when the routine is
dropped. If
automatic_sp_privileges is 0,
the server does not automatically add or drop these
privileges.
The creator of a routine is the account used to execute the
CREATE statement for it. This might not be
the same as the account named as the
DEFINER in the routine definition.
See also Section 18.2.2, “Stored Routines and MySQL Privileges”.
This variable was added in MySQL 5.0.3.
back_log
The number of outstanding connection requests MySQL can have.
This comes into play when the main MySQL thread gets very many
connection requests in a very short time. It then takes some
time (although very little) for the main thread to check the
connection and start a new thread. The
back_log value indicates how
many requests can be stacked during this short time before
MySQL momentarily stops answering new requests. You need to
increase this only if you expect a large number of connections
in a short period of time.
In other words, this value is the size of the listen queue for
incoming TCP/IP connections. Your operating system has its own
limit on the size of this queue. The manual page for the Unix
listen() system call should have more
details. Check your OS documentation for the maximum value for
this variable. back_log
cannot be set higher than your operating system limit.
basedir
The MySQL installation base directory. This variable can be
set with the --basedir option.
Relative path names for other variables usually are resolved
relative to the base directory.
bdb_cache_size
The size of the buffer that is allocated for caching indexes
and rows for BDB tables. If you don't use
BDB tables, you should start
mysqld with
--skip-bdb to not allocate
memory for this cache.
bdb_home
The base directory for BDB tables. This
should be assigned the same value as the
datadir variable.
bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes
and rows for BDB tables. If you don't use
BDB tables, you should set this to 0 or
start mysqld with
--skip-bdb to not allocate
memory for this cache.
bdb_logdir
The directory where the BDB storage engine
writes its log files. This variable can be set with the
--bdb-logdir option.
bdb_max_lock
The maximum number of locks that can be active for a
BDB table (10,000 by default). You should
increase this value if errors such as the following occur when
you perform long transactions or when
mysqld has to examine many rows to
calculate a query:
bdb: Lock table is out of available locks
Got error 12 from ...
bdb_shared_data
This is ON if you are using
--bdb-shared-data to start
Berkeley DB in multi-process mode. (Do not use
DB_PRIVATE when initializing Berkeley DB.)
bdb_tmpdir
The BDB temporary file directory.
binlog_cache_size
The size of the cache to hold the SQL statements for the
binary log during a transaction. A binary log cache is
allocated for each client if the server supports any
transactional storage engines and if the server has the binary
log enabled (--log-bin option).
If you often use large, multiple-statement transactions, you
can increase this cache size to get more performance. The
Binlog_cache_use and
Binlog_cache_disk_use status
variables can be useful for tuning the size of this variable.
See Section 5.2.3, “The Binary Log”.
MySQL Enterprise
For recommendations on the optimum setting for
binlog_cache_size subscribe
to the MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
bulk_insert_buffer_size
MyISAM uses a special tree-like cache to
make bulk inserts faster for
INSERT ...
SELECT, INSERT ... VALUES (...), (...),
..., and
LOAD DATA
INFILE when adding data to nonempty tables. This
variable limits the size of the cache tree in bytes per
thread. Setting it to 0 disables this optimization. The
default value is 8MB.
character_set_client
The character set for statements that arrive from the client.
The session value of this variable is set using the character
set requested by the client when the client connects to the
server. (Many clients support a
--default-character-set option to enable this
character set to be specified explicitly. See also
Section 9.1.4, “Connection Character Sets and Collations”.) The global value of the
variable is used to set the session value in cases when the
client-requested value is unknown or not available, or the
server is configured to ignore client requests:
The client is from a version of MySQL older than MySQL
4.1, and thus does not request a character set.
The client requests a character set not known to the
server. For example, a Japanese-enabled client requests
sjis when connecting to a server not
configured with sjis support.
mysqld was started with the
--skip-character-set-client-handshake
option, which causes it to ignore client character set
configuration. This reproduces MySQL 4.0 behavior and is
useful should you wish to upgrade the server without
upgrading all the clients.
character_set_connection
The character set used for literals that do not have a
character set introducer and for number-to-string conversion.
character_set_database
The character set used by the default database. The server
sets this variable whenever the default database changes. If
there is no default database, the variable has the same value
as character_set_server.
character_set_filesystem
The file system character set. This variable is used to
interpret string literals that refer to file names, such as in
the LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE statements and the
LOAD_FILE() function. Such file
names are converted from
character_set_client to
character_set_filesystem
before the file opening attempt occurs. The default value is
binary, which means that no conversion
occurs. For systems on which multi-byte file names are
allowed, a different value may be more appropriate. For
example, if the system represents file names using UTF-8, set
character_set_filesystem to
'utf8'. This variable was added in MySQL
5.0.19.
character_set_results
The character set used for returning query results such as
result sets or error messages to the client.
character_set_server
The server's default character set.
character_set_system
The character set used by the server for storing identifiers.
The value is always utf8.
character_sets_dir
The directory where character sets are installed.
collation_connection
The collation of the connection character set.
collation_database
The collation used by the default database. The server sets
this variable whenever the default database changes. If there
is no default database, the variable has the same value as
collation_server.
collation_server
The server's default collation.
completion_type
The transaction completion type:
If the value is 0 (the default),
COMMIT and
ROLLBACK
are unaffected.
If the value is 1, COMMIT
and
ROLLBACK
are equivalent to COMMIT AND CHAIN and
ROLLBACK AND CHAIN, respectively. (A
new transaction starts immediately with the same isolation
level as the just-terminated transaction.)
If the value is 2, COMMIT
and
ROLLBACK
are equivalent to COMMIT RELEASE and
ROLLBACK RELEASE, respectively. (The
server disconnects after terminating the transaction.)
This variable was added in MySQL 5.0.3
concurrent_insert
If 1 (the default), MySQL allows
INSERT and
SELECT statements to run
concurrently for MyISAM tables that have no
free blocks in the middle of the data file. You can turn this
option off by starting mysqld with
--safe-mode or
--skip-new.
In MySQL 5.0.6, this variable was changed to take three
integer values:
See also Section 7.3.3, “Concurrent Inserts”.
connect_timeout
The number of seconds that the mysqld
server waits for a connect packet before responding with
Bad handshake. The default value is 10
seconds as of MySQL 5.0.52 and 5 seconds before that.
Increasing the
connect_timeout value might
help if clients frequently encounter errors of the form
Lost connection to MySQL server at
'XXX', system error:
errno.
datadir
The MySQL data directory. This variable can be set with the
--datadir option.
date_format
This variable is unused.
datetime_format
This variable is unused.
default_week_format
The default mode value to use for the
WEEK() function. See
Section 11.6, “Date and Time Functions”.
delay_key_write
This option applies only to MyISAM tables.
It can have one of the following values to affect handling of
the DELAY_KEY_WRITE table option that can
be used in CREATE TABLE
statements.
If DELAY_KEY_WRITE is enabled for a table,
the key buffer is not flushed for the table on every index
update, but only when the table is closed. This speeds up
writes on keys a lot, but if you use this feature, you should
add automatic checking of all MyISAM tables
by starting the server with the
--myisam-recover option (for
example,
--myisam-recover=BACKUP,FORCE).
See Section 5.1.2, “Server Command Options”, and
Section 13.1.1, “MyISAM Startup Options”.
Warning
If you enable external locking with
--external-locking, there is
no protection against index corruption for tables that use
delayed key writes.
delayed_insert_limit
After inserting
delayed_insert_limit delayed
rows, the INSERT DELAYED
handler thread checks whether there are any
SELECT statements pending. If
so, it allows them to execute before continuing to insert
delayed rows.
delayed_insert_timeout
How many seconds an INSERT
DELAYED handler thread should wait for
INSERT statements before
terminating.
delayed_queue_size
This is a per-table limit on the number of rows to queue when
handling INSERT DELAYED
statements. If the queue becomes full, any client that issues
an INSERT DELAYED statement
waits until there is room in the queue again.
div_precision_increment
This variable indicates the number of digits by which to
increase the scale of the result of division operations
performed with the
/ operator.
The default value is 4. The minimum and maximum values are 0
and 30, respectively. The following example illustrates the
effect of increasing the default value.
mysql> SELECT 1/7;
+--------+
| 1/7 |
+--------+
| 0.1429 |
+--------+
mysql> SET div_precision_increment = 12;
mysql> SELECT 1/7;
+----------------+
| 1/7 |
+----------------+
| 0.142857142857 |
+----------------+
This variable was added in MySQL 5.0.6.
engine_condition_pushdown
When the value of this variable is 0 (OFF),
a query such as SELECT * FROM t WHERE mycol =
42, where mycol is a nonindexed
column, is executed as a full table scan. The storage engine
sends every row to the MySQL server, which applies the
WHERE condition. If
engine_condition_pushdown is
set to 1 (ON), the condition is
“pushed down” to the storage engine, which uses
the condition to perform the scan, and sends back to the MySQL
server only those rows that match the condition. By default,
this variable is OFF.
In MySQL 5.0, this variable is useful only with
the NDBCLUSTER storage engine.
However, we intend to implement it for additional storage
engines in future MySQL releases.
Setting this variable to ON on a MySQL
Server acting as a MySQL Cluster SQL node causes
WHERE conditions on unindexed columns to be
evaluated on the cluster's data nodes and only the rows that
match to be sent back to the SQL node that issued the query.
This means the amount of cluster data that must be sent over
the network is greatly reduced, increasing the efficiency with
which results are returned.
For more information, see
Section 7.2.7, “Condition Pushdown Optimization”.
This variable was added in MySQL 5.0.3.
expire_logs_days
The number of days for automatic binary log file removal. The
default is 0, which means “no automatic removal.”
Possible removals happen at startup and when the binary log is
flushed. Log flushing occurs as indicated in
Section 5.2, “MySQL Server Logs”.
To remove binary log files manually, use the
PURGE BINARY LOGS statement.
See Section 12.6.1.1, “PURGE BINARY LOGS Syntax”.
flush
If ON, the server flushes (synchronizes)
all changes to disk after each SQL statement. Normally, MySQL
does a write of all changes to disk only after each SQL
statement and lets the operating system handle the
synchronizing to disk. See Section B.5.4.2, “What to Do If MySQL Keeps Crashing”. This
variable is set to ON if you start
mysqld with the
--flush option.
flush_time
If this is set to a nonzero value, all tables are closed every
flush_time seconds to free up
resources and synchronize unflushed data to disk. This option
is best used only on Windows 9x or Me, or on systems with
minimal resources.
ft_boolean_syntax
The list of operators supported by boolean full-text searches
performed using IN BOOLEAN MODE. See
Section 11.8.2, “Boolean Full-Text Searches”.
The default variable value is
'+ -><()~*:""&|'. The rules
for changing the value are as follows:
Operator function is determined by position within the
string.
The replacement value must be 14 characters.
Each character must be an ASCII nonalphanumeric character.
Either the first or second character must be a space.
No duplicates are allowed except the phrase quoting
operators in positions 11 and 12. These two characters are
not required to be the same, but they are the only two
that may be.
Positions 10, 13, and 14 (which by default are set to
“:”,
“&”, and
“|”) are reserved for
future extensions.
ft_max_word_len
The maximum length of the word to be included in a
FULLTEXT index.
Note
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
tbl_name QUICK.
ft_min_word_len
The minimum length of the word to be included in a
FULLTEXT index.
Note
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
tbl_name QUICK.
ft_query_expansion_limit
The number of top matches to use for full-text searches
performed using WITH QUERY EXPANSION.
ft_stopword_file
The file from which to read the list of stopwords for
full-text searches. The server looks for the file in the data
directory unless an absolute path name is given to specify a
different directory. All the words from the file are used;
comments are not honored. By default, a
built-in list of stopwords is used (as defined in the
myisam/ft_static.c file). Setting this
variable to the empty string ('') disables
stopword filtering.
Note
FULLTEXT indexes must be rebuilt after
changing this variable or the contents of the stopword file.
Use REPAIR TABLE
tbl_name QUICK.
group_concat_max_len
The maximum allowed result length in bytes for the
GROUP_CONCAT() function. The
default is 1024.
have_archive
YES if mysqld supports
ARCHIVE tables, NO if
not.
have_bdb
YES if mysqld supports
BDB tables. DISABLED if
--skip-bdb is used.
have_blackhole_engine
YES if mysqld supports
BLACKHOLE tables, NO if
not.
have_compress
YES if the zlib
compression library is available to the server,
NO if not. If not, the
COMPRESS() and
UNCOMPRESS() functions cannot
be used.
have_crypt
YES if the crypt()
system call is available to the server, NO
if not. If not, the ENCRYPT()
function cannot be used.
have_csv
YES if mysqld supports
CSV tables, NO if not.
have_example_engine
YES if mysqld supports
EXAMPLE tables, NO if
not.
have_federated_engine
YES if mysqld supports
FEDERATED tables, NO if
not. This variable was added in MySQL 5.0.3.
have_geometry
YES if the server supports spatial data
types, NO if not.
have_innodb
YES if mysqld supports
InnoDB tables. DISABLED
if
--skip-innodb
is used.
have_isam
In MySQL 5.0, this variable appears only for
reasons of backward compatibility. It is always
NO because ISAM tables
are no longer supported.
have_merge_engine
YES if mysqld supports
MERGE tables. DISABLED
if --skip-merge is used. This
variable was added in MySQL 5.0.24.
have_openssl
YES if mysqld supports
SSL connections, NO if not. As of MySQL
5.0.38, this variable is an alias for
have_ssl.
have_query_cache
YES if mysqld supports
the query cache, NO if not.
have_raid
In MySQL 5.0, this variable appears only for
reasons of backward compatibility. It is always
NO because RAID tables
are no longer supported.
have_rtree_keys
YES if RTREE indexes are
available, NO if not. (These are used for
spatial indexes in MyISAM tables.)
have_ssl
YES if mysqld supports
SSL connections, NO if not. This variable
was added in MySQL 5.0.38. Before that, use
have_openssl.
have_symlink
YES if symbolic link support is enabled,
NO if not. This is required on Unix for
support of the DATA DIRECTORY and
INDEX DIRECTORY table options, and on
Windows for support of data directory symlinks.
hostname
The server sets this variable to the server host name at
startup. This variable was added in MySQL 5.0.38.
init_connect
A string to be executed by the server for each client that
connects. The string consists of one or more SQL statements.
To specify multiple statements, separate them by semicolon
characters. For example, each client begins by default with
autocommit mode enabled. There is no global system variable to
specify that autocommit should be disabled by default, but
init_connect can be used to
achieve the same effect:
SET GLOBAL init_connect='SET autocommit=0';
This variable can also be set on the command line or in an
option file. To set the variable as just shown using an option
file, include these lines:
[mysqld]
init_connect='SET autocommit=0'
Note that the content of
init_connect is not executed
for users that have the SUPER
privilege. This is done so that an erroneous value for
init_connect does not prevent
all clients from connecting. For example, the value might
contain a statement that has a syntax error, thus causing
client connections to fail. Not executing
init_connect for users that
have the SUPER privilege
enables them to open a connection and fix the
init_connect value.
init_file
The name of the file specified with the
--init-file option when you
start the server. This should be a file containing SQL
statements that you want the server to execute when it starts.
Each statement must be on a single line and should not include
comments. No statement terminator such as
;, \g, or
\G should be given at the end of each
statement.
Note that the --init-file
option is unavailable if MySQL was configured with the
--disable-grant-options
option. See Section 2.16.2, “Typical configure Options”.
innodb_xxx
InnoDB system variables are listed in
Section 13.2.3, “InnoDB Startup Options and System Variables”.
interactive_timeout
The number of seconds the server waits for activity on an
interactive connection before closing it. An interactive
client is defined as a client that uses the
CLIENT_INTERACTIVE option to
mysql_real_connect(). See also
wait_timeout.
join_buffer_size
The size of the buffer that is used for plain index scans,
range index scans, and joins that do not use indexes and thus
perform full table scans. Normally, the best way to get fast
joins is to add indexes. Increase the value of
join_buffer_size to get a
faster full join when adding indexes is not possible. One join
buffer is allocated for each full join between two tables. For
a complex join between several tables for which indexes are
not used, multiple join buffers might be necessary.
The maximum allowable setting for
join_buffer_size is 4GB.
keep_files_on_create
If a MyISAM table is created with no
DATA DIRECTORY option, the
.MYD file is created in the database
directory. By default, if MyISAM finds an
existing .MYD file in this case, it
overwrites it. The same applies to .MYI
files for tables created with no INDEX
DIRECTORY option. To suppress this behavior, set the
keep_files_on_create variable
to ON (1), in which case
MyISAM will not overwrite existing files
and returns an error instead. The default value is
OFF (0).
If a MyISAM table is created with a
DATA DIRECTORY or INDEX
DIRECTORY option and an existing
.MYD or .MYI file is
found, MyISAM always returns an error. It will not overwrite a
file in the specified directory.
This variable was added in MySQL 5.0.48.
key_buffer_size
Index blocks for MyISAM tables are buffered
and are shared by all threads.
key_buffer_size is the size
of the buffer used for index blocks. The key buffer is also
known as the key cache.
The maximum allowable setting for
key_buffer_size is 4GB on
32-bit platforms. As of MySQL 5.0.52, values larger than 4GB
are allowed for 64-bit platforms (except 64-bit Windows, for
which large values are truncated to 4GB with a warning). The
effective maximum size might be less, depending on your
available physical RAM and per-process RAM limits imposed by
your operating system or hardware platform. The value of this
variable indicates the amount of memory requested. Internally,
the server allocates as much memory as possible up to this
amount, but the actual allocation might be less.
You can increase the value to get better index handling for
all reads and multiple writes; on a system whose primary
function is to run MySQL using the
MyISAM storage engine, 25% of the
machine's total memory is an acceptable value for this
variable. However, you should be aware that, if you make the
value too large (for example, more than 50% of the
machine's total memory), your system might start to page
and become extremely slow. This is because MySQL relies on the
operating system to perform file system caching for data
reads, so you must leave some room for the file system cache.
You should also consider the memory requirements of any other
storage engines that you may be using in addition to
MyISAM.
For even more speed when writing many rows at the same time,
use LOCK TABLES. See
Section 7.2.19, “Speed of INSERT Statements”.
You can check the performance of the key buffer by issuing a
SHOW STATUS statement and
examining the
Key_read_requests,
Key_reads,
Key_write_requests, and
Key_writes status variables.
(See Section 12.5.5, “SHOW Syntax”.) The
Key_reads/Key_read_requests ratio should
normally be less than 0.01. The
Key_writes/Key_write_requests ratio is
usually near 1 if you are using mostly updates and deletes,
but might be much smaller if you tend to do updates that
affect many rows at the same time or if you are using the
DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using
key_buffer_size in
conjunction with the
Key_blocks_unused status
variable and the buffer block size, which is available from
the key_cache_block_size
system variable:
1 - ((Key_blocks_unused ? key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key
buffer may be allocated internally for administrative
structures.
It is possible to create multiple MyISAM
key caches. The size limit of 4GB applies to each cache
individually, not as a group. See
Section 7.4.5, “The MyISAM Key Cache”.
key_cache_age_threshold
This value controls the demotion of buffers from the hot
sublist of a key cache to the warm sublist. Lower values cause
demotion to happen more quickly. The minimum value is 100. The
default value is 300. See Section 7.4.5, “The MyISAM Key Cache”.
key_cache_block_size
The size in bytes of blocks in the key cache. The default
value is 1024. See Section 7.4.5, “The MyISAM Key Cache”.
key_cache_division_limit
The division point between the hot and warm sublists of the
key cache buffer list. The value is the percentage of the
buffer list to use for the warm sublist. Allowable values
range from 1 to 100. The default value is 100. See
Section 7.4.5, “The MyISAM Key Cache”.
language
The directory where error messages are located. See
Section 9.3, “Setting the Error Message Language”.
large_files_support
Whether mysqld was compiled with options
for large file support.
large_pages
Whether large page support is enabled (via the
--large-pages option). See
Section 7.5.9, “Enabling Large Page Support”. This variable was added
in MySQL 5.0.3.
For more information, see
the entry for the
--large-pages server
option.
large_page_size
If large page support is enabled, this shows the size of
memory pages. Currently, large memory pages are supported only
on Linux; on other platforms, the value of this variable is
always 0. This variable was added in MySQL 5.0.3.
For more information, see
the entry for the
--large-pages server
option.
lc_time_names
This variable specifies the locale that controls the language
used to display day and month names and abbreviations. This
variable affects the output from the
DATE_FORMAT(),
DAYNAME() and
MONTHNAME() functions. Locale
names are POSIX-style values such as
'ja_JP' or 'pt_BR'. The
default value is 'en_US' regardless of your
system's locale setting. For further information, see
Section 9.8, “MySQL Server Locale Support”. This variable was added in
MySQL 5.0.25.
license
The type of license the server has.
local_infile
Whether LOCAL is supported for
LOAD DATA
INFILE statements. See
Section 5.3.4, “Security Issues with LOAD
DATA LOCAL”.
locked_in_memory
Whether mysqld was locked in memory with
--memlock.
log
Whether logging of all statements to the general query log is
enabled. See Section 5.2.2, “The General Query Log”.
log_bin
Whether the binary log is enabled. See
Section 5.2.3, “The Binary Log”.
log_bin_trust_function_creators
This variable applies when binary logging is enabled. It
controls whether stored function creators can be trusted not
to create stored functions that will cause unsafe events to be
written to the binary log. If set to 0 (the default), users
are not allowed to create or alter stored functions unless
they have the SUPER privilege
in addition to the CREATE
ROUTINE or ALTER
ROUTINE privilege. A setting of 0 also enforces the
restriction that a function must be declared with the
DETERMINISTIC characteristic, or with the
READS SQL DATA or NO SQL
characteristic. If the variable is set to 1, MySQL does not
enforce these restrictions on stored function creation. This
variable also applies to trigger creation. See
Section 18.5, “Binary Logging of Stored Programs”.
This variable was added in MySQL 5.0.16.
log_bin_trust_routine_creators
This is the old name for
log_bin_trust_function_creators.
Before MySQL 5.0.16, it also applies to stored procedures, not
just stored functions. As of 5.0.16, this variable is
deprecated. It is recognized for backward compatibility but
its use results in a warning.
This variable was added in MySQL 5.0.6.
log_error
The location of the error log.
log_queries_not_using_indexes
Whether queries that do not use indexes are logged to the slow
query log. See Section 5.2.4, “The Slow Query Log”. This variable
was added in MySQL 5.0.23.
log_slow_queries
Whether slow queries should be logged. “Slow” is
determined by the value of the
long_query_time variable. See
Section 5.2.4, “The Slow Query Log”.
log_warnings
Whether to produce additional warning messages. It is enabled
(1) by default and can be disabled by setting it to 0. Aborted
connections are not logged to the error log unless the value
is greater than 1.
long_query_time
If a query takes longer than this many seconds, the server
increments the Slow_queries
status variable. If you are using the
--log-slow-queries option, the
query is logged to the slow query log file. This value is
measured in real time, not CPU time, so a query that is under
the threshold on a lightly loaded system might be above the
threshold on a heavily loaded one. The minimum value is 1. The
default is 10. See Section 5.2.4, “The Slow Query Log”.
low_priority_updates
If set to 1, all
INSERT,
UPDATE,
DELETE, and LOCK TABLE
WRITE statements wait until there is no pending
SELECT or LOCK TABLE
READ on the affected table. This affects only
storage engines that use only table-level locking (such as
MyISAM, MEMORY, and
MERGE). This variable previously was named
sql_low_priority_updates.
lower_case_file_system
This variable describes the case sensitivity of file names on
the file system where the data directory is located.
OFF means file names are case sensitive,
ON means they are not case sensitive.
lower_case_table_names
If set to 1, table names are stored in lowercase on disk and
table name comparisons are not case sensitive. If set to 2
table names are stored as given but compared in lowercase.
This option also applies to database names and table aliases.
See Section 8.2.2, “Identifier Case Sensitivity”.
If you are using InnoDB tables, you should
set this variable to 1 on all platforms to force names to be
converted to lowercase.
You should not set this variable to 0 if
you are running MySQL on a system that does not have
case-sensitive file names (such as Windows or Mac OS X). If
this variable is not set at startup and the file system on
which the data directory is located does not have
case-sensitive file names, MySQL automatically sets
lower_case_table_names to 2.
max_allowed_packet
The maximum size of one packet or any generated/intermediate
string.
The packet message buffer is initialized to
net_buffer_length bytes, but
can grow up to
max_allowed_packet bytes when
needed. This value by default is small, to catch large
(possibly incorrect) packets.
You must increase this value if you are using large
BLOB columns or long strings.
It should be as big as the largest
BLOB you want to use. The
protocol limit for
max_allowed_packet is 1GB.
The value should be a multiple of 1024; nonmultiples are
rounded down to the nearest multiple.
When you change the message buffer size by changing the value
of the max_allowed_packet
variable, you should also change the buffer size on the client
side if your client program allows it. On the client side,
max_allowed_packet has a
default of 1GB. Some programs such as mysql
and mysqldump enable you to change the
client-side value by setting
max_allowed_packet on the
command line or in an option file.
As of MySQL 5.0.84, the session value of this variable is read
only. Before 5.0.84, setting the session value is allowed but
has no effect.
max_connect_errors
If there are more than this number of interrupted connections
from a host, that host is blocked from further connections.
You can unblock blocked hosts with the
FLUSH HOSTS
statement. If a connection is established successfully within
fewer than max_connect_errors
attempts after a previous connection was interrupted, the
error count for the host is cleared to zero. However, once a
host is blocked, the
FLUSH HOSTS
statement is the only way to unblock it.
max_connections
The number of simultaneous client connections allowed. By
default, this is 100. See
Section B.5.2.7, “Too many connections”, for more information.
MySQL Enterprise
For notification that the maximum number of connections is
getting dangerously high and for advice on setting the
optimum value for
max_connections subscribe
to the MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
Increasing this value increases the number of file descriptors
that mysqld requires. See
Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor
limits.
max_delayed_threads
Do not start more than this number of threads to handle
INSERT DELAYED statements. If
you try to insert data into a new table after all
INSERT DELAYED threads are in
use, the row is inserted as if the DELAYED
attribute wasn't specified. If you set this to 0, MySQL never
creates a thread to handle DELAYED rows; in
effect, this disables DELAYED entirely.
For the SESSION value of this variable, the
only valid values are 0 or the GLOBAL
value.
max_error_count
The maximum number of error, warning, and note messages to be
stored for display by the SHOW
ERRORS and SHOW
WARNINGS statements.
max_heap_table_size
This variable sets the maximum size to which
MEMORY tables are allowed to grow. The
value of the variable is used to calculate
MEMORY table MAX_ROWS
values. Setting this variable has no effect on any existing
MEMORY table, unless the table is
re-created with a statement such as
CREATE TABLE or altered with
ALTER TABLE or
TRUNCATE TABLE. A server
restart also sets the maximum size of existing
MEMORY tables to the global
max_heap_table_size value.
Note
On 64-bit platforms, the maximum value for this variable is
1844674407370954752.
MySQL Enterprise
Subscribers to the MySQL Enterprise Monitor receive
recommendations for the optimum setting for
max_heap_table_size. For
more information, see
http://www.mysql.com/products/enterprise/advisors.html.
max_insert_delayed_threads
This variable is a synonym for
max_delayed_threads.
max_join_size
Do not allow SELECT statements
that probably need to examine more than
max_join_size rows (for
single-table statements) or row combinations (for
multiple-table statements) or that are likely to do more than
max_join_size disk seeks. By
setting this value, you can catch
SELECT statements where keys
are not used properly and that would probably take a long
time. Set it if your users tend to perform joins that lack a
WHERE clause, that take a long time, or
that return millions of rows.
Setting this variable to a value other than
DEFAULT resets the value of
sql_big_selects to
0. If you set the
sql_big_selects value again,
the max_join_size variable is
ignored.
If a query result is in the query cache, no result size check
is performed, because the result has previously been computed
and it does not burden the server to send it to the client.
This variable previously was named
sql_max_join_size.
max_length_for_sort_data
The cutoff on the size of index values that determines which
filesort algorithm to use. See
Section 7.2.13, “ORDER BY Optimization”.
max_prepared_stmt_count
This variable limits the total number of prepared statements
in the server. It can be used in environments where there is
the potential for denial-of-service attacks based on running
the server out of memory by preparing huge numbers of
statements. If the value is set lower than the current number
of prepared statements, existing statements are not affected
and can be used, but no new statements can be prepared until
the current number drops below the limit. The default value is
16,382. The allowable range of values is from 0 to 1 million.
Setting the value to 0 disables prepared statements. This
variable was added in MySQL 5.0.21.
max_relay_log_size
If a write by a replication slave to its relay log causes the
current log file size to exceed the value of this variable,
the slave rotates the relay logs (closes the current file and
opens the next one). If
max_relay_log_size is 0, the
server uses max_binlog_size
for both the binary log and the relay log. If
max_relay_log_size is greater
than 0, it constrains the size of the relay log, which enables
you to have different sizes for the two logs. You must set
max_relay_log_size to between
4096 bytes and 1GB (inclusive), or to 0. The default value is
0. See Section 16.4.1, “Replication Implementation Details”.
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows
based on a key. The MySQL optimizer assumes that no more than
this number of key seeks are required when searching for
matching rows in a table by scanning an index, regardless of
the actual cardinality of the index (see
Section 12.5.5.18, “SHOW INDEX Syntax”). By setting this to a low value
(say, 100), you can force MySQL to prefer indexes instead of
table scans.
max_sort_length
The number of bytes to use when sorting
BLOB or
TEXT values. Only the first
max_sort_length bytes of each
value are used; the rest are ignored.
max_sp_recursion_depth
The number of times that any given stored procedure may be
called recursively. The default value for this option is 0,
which completely disallows recursion in stored procedures. The
maximum value is 255.
Stored procedure recursion increases the demand on thread
stack space. If you increase the value of
max_sp_recursion_depth, it
may be necessary to increase thread stack size by increasing
the value of thread_stack at
server startup.
This variable was added in MySQL 5.0.17.
max_tmp_tables
The maximum number of temporary tables a client can keep open
at the same time. (This variable does not yet do anything.)
max_user_connections
The maximum number of simultaneous connections allowed to any
given MySQL account. A value of 0 means “no
limit.”
Before MySQL 5.0.3, this variable has only global scope.
Beginning with MySQL 5.0.3, it also has a read-only session
scope. The session variable has the same value as the global
variable unless the current account has a nonzero
MAX_USER_CONNECTIONS resource limit. In
that case, the session value reflects the account limit.
max_write_lock_count
After this many write locks, allow some pending read lock
requests to be processed in between.
myisam_data_pointer_size
The default pointer size in bytes, to be used by
CREATE TABLE for
MyISAM tables when no
MAX_ROWS option is specified. This variable
cannot be less than 2 or larger than 7. The default value is 6
(4 before MySQL 5.0.6). See Section B.5.2.12, “The table is full”.
myisam_max_extra_sort_file_size
(DEPRECATED)
This variable is not used. It was removed in MySQL 5.0.6.
myisam_max_sort_file_size
The maximum size of the temporary file that MySQL is allowed
to use while re-creating a MyISAM index
(during REPAIR TABLE,
ALTER TABLE, or
LOAD DATA
INFILE). If the file size would be larger than this
value, the index is created using the key cache instead, which
is slower. The value is given in bytes.
The default value is 2GB. If MyISAM index
files exceed this size and disk space is available, increasing
the value may help performance.
myisam_mmap_size
The maximum amount of memory to use for memory mapping
compressed MyISAM files. If many
compressed MyISAM tables are used, the
value can be decreased to reduce the likelihood of
memory-swapping problems. This variable was added in MySQL
5.0.90.
myisam_recover_options
The value of the
--myisam-recover option. See
Section 5.1.2, “Server Command Options”.
myisam_repair_threads
If this value is greater than 1, MyISAM
table indexes are created in parallel (each index in its own
thread) during the Repair by sorting
process. The default value is 1.
Note
Multi-threaded repair is still
beta-quality code.
myisam_sort_buffer_size
The size of the buffer that is allocated when sorting
MyISAM indexes during a
REPAIR TABLE or when creating
indexes with CREATE INDEX or
ALTER TABLE.
The maximum allowable setting for
myisam_sort_buffer_size is
4GB.
myisam_stats_method
How the server treats NULL values when
collecting statistics about the distribution of index values
for MyISAM tables. This variable has three
possible values, nulls_equal,
nulls_unequal, and
nulls_ignored. For
nulls_equal, all NULL
index values are considered equal and form a single value
group that has a size equal to the number of
NULL values. For
nulls_unequal, NULL
values are considered unequal, and each
NULL forms a distinct value group of size
1. For nulls_ignored,
NULL values are ignored.
The method that is used for generating table statistics
influences how the optimizer chooses indexes for query
execution, as described in
Section 7.4.7, “MyISAM Index Statistics Collection”.
Any unique prefix of a valid value may be used to set the
value of this variable.
This variable was added in MySQL 5.0.14. For older versions,
the statistics collection method is equivalent to
nulls_equal.
named_pipe
(Windows only.) Indicates whether the server supports
connections over named pipes.
net_buffer_length
Each client thread is associated with a connection buffer and
result buffer. Both begin with a size given by
net_buffer_length but are
dynamically enlarged up to
max_allowed_packet bytes as
needed. The result buffer shrinks to
net_buffer_length after each
SQL statement.
This variable should not normally be changed, but if you have
very little memory, you can set it to the expected length of
statements sent by clients. If statements exceed this length,
the connection buffer is automatically enlarged. The maximum
value to which
net_buffer_length can be set
is 1MB.
As of MySQL 5.0.84, the session value of this variable is read
only. Before 5.0.84, setting the session value is allowed but
has no effect.
net_read_timeout
The number of seconds to wait for more data from a connection
before aborting the read. This timeout applies only to TCP/IP
connections, not to connections made via Unix socket files,
named pipes, or shared memory. When the server is reading from
the client, net_read_timeout
is the timeout value controlling when to abort. When the
server is writing to the client,
net_write_timeout is the
timeout value controlling when to abort. See also
slave_net_timeout.
net_retry_count
If a read on a communication port is interrupted, retry this
many times before giving up. This value should be set quite
high on FreeBSD because internal interrupts are sent to all
threads.
net_write_timeout
The number of seconds to wait for a block to be written to a
connection before aborting the write. This timeout applies
only to TCP/IP connections, not to connections made via Unix
socket files, named pipes, or shared memory. See also
net_read_timeout.
new
This variable was used in MySQL 4.0 to turn on some 4.1
behaviors, and is retained for backward compatibility. In
MySQL 5.0, its value is always
OFF.
old_passwords
Whether the server should use pre-4.1-style passwords for
MySQL user accounts. See Section B.5.2.4, “Client does not support authentication protocol”.
one_shot
This is not a variable, but it can be used when setting some
variables. It is described in Section 12.5.4, “SET Syntax”.
open_files_limit
The number of files that the operating system allows
mysqld to open. This is the real value
allowed by the system and might be different from the value
you gave using the
--open-files-limit option to
mysqld or mysqld_safe.
The value is 0 on systems where MySQL can't change the number
of open files.
optimizer_prune_level
Controls the heuristics applied during query optimization to
prune less-promising partial plans from the optimizer search
space. A value of 0 disables heuristics so that the optimizer
performs an exhaustive search. A value of 1 causes the
optimizer to prune plans based on the number of rows retrieved
by intermediate plans. This variable was added in MySQL 5.0.1.
optimizer_search_depth
The maximum depth of search performed by the query optimizer.
Values larger than the number of relations in a query result
in better query plans, but take longer to generate an
execution plan for a query. Values smaller than the number of
relations in a query return an execution plan quicker, but the
resulting plan may be far from being optimal. If set to 0, the
system automatically picks a reasonable value. If set to 63,
the optimizer switches to the algorithm used in MySQL 5.0.0
(and previous versions) for performing searches. This variable
was added in MySQL 5.0.1.
pid_file
The path name of the process ID (PID) file. This variable can
be set with the --pid-file
option.
plugin_dir
The path name of the plugin directory. This variable was added
in MySQL 5.0.67. If the value is nonempty, user-defined
function object files must be located in this directory. If
the value is empty, the behavior that is used before 5.0.67
applies: The UDF object files must be located in a directory
that is searched by your system's dynamic linker.
port
The number of the port on which the server listens for TCP/IP
connections. This variable can be set with the
--port option.
preload_buffer_size
The size of the buffer that is allocated when preloading
indexes.
prepared_stmt_count
The current number of prepared statements. (The maximum number
of statements is given by the
max_prepared_stmt_count
system variable.) This variable was added in MySQL 5.0.21. In
MySQL 5.0.32, it was converted to the global
Prepared_stmt_count status
variable.
protocol_version
The version of the client/server protocol used by the MySQL
server.
pseudo_thread_id
This variable is for internal server use.
query_alloc_block_size
The allocation size of memory blocks that are allocated for
objects created during statement parsing and execution. If you
have problems with memory fragmentation, it might help to
increase this parameter.
query_cache_limit
Don't cache results that are larger than this number of bytes.
The default value is 1MB.
query_cache_min_res_unit
The minimum size (in bytes) for blocks allocated by the query
cache. The default value is 4096 (4KB). Tuning information for
this variable is given in
Section 7.5.5.3, “Query Cache Configuration”.
query_cache_size
The amount of memory allocated for caching query results. The
default value is 0, which disables the query cache. The
allowable values are multiples of 1024; other values are
rounded down to the nearest multiple. Note that
query_cache_size bytes of
memory are allocated even if
query_cache_type is set to 0.
See Section 7.5.5.3, “Query Cache Configuration”, for more
information.
The query cache needs a minimum size of about 40KB to allocate
its structures. (The exact size depends on system
architecture.) If you set the value of
query_cache_size too small,
you'll get a warning, as described in
Section 7.5.5.3, “Query Cache Configuration”.
query_cache_type
Set the query cache type. Setting the
GLOBAL value sets the type for all clients
that connect thereafter. Individual clients can set the
SESSION value to affect their own use of
the query cache. Possible values are shown in the following
table.
This variable defaults to ON.
Any unique prefix of a valid value may be used to set the
value of this variable.
query_cache_wlock_invalidate
Normally, when one client acquires a WRITE
lock on a MyISAM table, other clients are
not blocked from issuing statements that read from the table
if the query results are present in the query cache. Setting
this variable to 1 causes acquisition of a
WRITE lock for a table to invalidate any
queries in the query cache that refer to the table. This
forces other clients that attempt to access the table to wait
while the lock is in effect.
query_prealloc_size
The size of the persistent buffer used for statement parsing
and execution. This buffer is not freed between statements. If
you are running complex queries, a larger
query_prealloc_size value
might be helpful in improving performance, because it can
reduce the need for the server to perform memory allocation
during query execution operations.
range_alloc_block_size
The size of blocks that are allocated when doing range
optimization.
read_buffer_size
Each thread that does a sequential scan allocates a buffer of
this size (in bytes) for each table it scans. If you do many
sequential scans, you might want to increase this value, which
defaults to 131072. The value of this variable should be a
multiple of 4KB. If it is set to a value that is not a
multiple of 4KB, its value will be rounded down to the nearest
multiple of 4KB.
The maximum allowable setting for
read_buffer_size is 2GB.
read_buffer_size and
read_rnd_buffer_size are not
specific to any storage engine and apply in a general manner
for optimization. See Section 7.5.8, “How MySQL Uses Memory”, for
example.
read_only
This variable is off by default. When it is enabled, the
server allows no updates except from users that have the
SUPER privilege or (on a slave
server) from updates performed by slave threads. On a slave
server, this can be useful to ensure that the slave accepts
updates only from its master server and not from clients. As
of MySQL 5.0.16, this variable does not apply to
TEMPORARY tables.
read_only exists only as a
GLOBAL variable, so changes to its value
require the SUPER privilege.
Changes to read_only on a
master server are not replicated to slave servers. The value
can be set on a slave server independent of the setting on the
master.
read_rnd_buffer_size
When reading rows in sorted order following a key-sorting
operation, the rows are read through this buffer to avoid disk
seeks. See Section 7.2.13, “ORDER BY Optimization”. Setting
the variable to a large value can improve ORDER
BY performance by a lot. However, this is a buffer
allocated for each client, so you should not set the global
variable to a large value. Instead, change the session
variable only from within those clients that need to run large
queries.
The maximum allowable setting for
read_rnd_buffer_size is 2GB.
read_buffer_size and
read_rnd_buffer_size are not
specific to any storage engine and apply in a general manner
for optimization. See Section 7.5.8, “How MySQL Uses Memory”, for
example.
relay_log_purge
Disables or enables automatic purging of relay log files as
soon as they are not needed any more. The default value is 1
(ON).
relay_log_space_limit
The maximum amount of space to use for all relay logs.
secure_auth
If the MySQL server has been started with the
--secure-auth option, it blocks
connections from all accounts that have passwords stored in
the old (pre-4.1) format. In that case, the value of this
variable is ON, otherwise it is
OFF.
You should enable this option if you want to prevent all use
of passwords employing the old format (and hence insecure
communication over the network).
Server startup fails with an error if this option is enabled
and the privilege tables are in pre-4.1 format. See
Section B.5.2.4, “Client does not support authentication protocol”.
secure_file_priv
By default, this variable is empty. If set to the name of a
directory, it limits the effect of the
LOAD_FILE() function and the
LOAD DATA and
SELECT ... INTO
OUTFILE statements to work only with files in that
directory.
This variable was added in MySQL 5.0.38.
server_id
The server ID, used in replication to give each master and
slave a unique identity. This variable is set by the
--server-id option. For each
server participating in replication, you should pick a
positive integer in the range from 1 to
232 – 1 to act as that
server's ID.
shared_memory
(Windows only.) Whether the server allows shared-memory
connections.
shared_memory_base_name
(Windows only.) The name of shared memory to use for
shared-memory connections. This is useful when running
multiple MySQL instances on a single physical machine. The
default name is MYSQL. The name is case
sensitive.
skip_external_locking
This is OFF if mysqld
uses external locking, ON if external
locking is disabled.
skip_networking
This is ON if the server allows only local
(non-TCP/IP) connections. On Unix, local connections use a
Unix socket file. On Windows, local connections use a named
pipe or shared memory. On NetWare, only TCP/IP connections are
supported, so do not set this variable to
ON. This variable can be set to
ON with the
--skip-networking option.
skip_show_database
This prevents people from using the SHOW
DATABASES statement if they do not have the
SHOW DATABASES privilege. This
can improve security if you have concerns about users being
able to see databases belonging to other users. Its effect
depends on the SHOW DATABASES
privilege: If the variable value is ON, the
SHOW DATABASES statement is
allowed only to users who have the SHOW
DATABASES privilege, and the statement displays all
database names. If the value is OFF,
SHOW DATABASES is allowed to
all users, but displays the names of only those databases for
which the user has the SHOW
DATABASES or other privilege.
slow_launch_time
If creating a thread takes longer than this many seconds, the
server increments the
Slow_launch_threads status
variable.
socket
On Unix platforms, this variable is the name of the socket
file that is used for local client connections. The default is
/tmp/mysql.sock. (For some distribution
formats, the directory might be different, such as
/var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe that
is used for local client connections. The default value is
MySQL (not case sensitive).
sort_buffer_size
Each thread that needs to do a sort allocates a buffer of this
size. Increase this value for faster ORDER
BY or GROUP BY operations. See
Section B.5.4.4, “Where MySQL Stores Temporary Files”.
The maximum allowable setting for
sort_buffer_size is 4GB.
sql_mode
The current server SQL mode, which can be set dynamically. See
Section 5.1.7, “Server SQL Modes”.
sql_select_limit
The maximum number of rows to return from
SELECT statements. The default
value for a new connection is the maximum number of rows that
the server allows per table, which depends on the server
configuration and may be affected if the server build was
configured with
--with-big-tables. Typical
default values are (232)–1 or
(264)–1. If you have changed
the limit, the default value can be restored by assigning a
value of DEFAULT.
If a SELECT has a
LIMIT clause, the LIMIT
takes precedence over the value of
sql_select_limit.
sql_select_limit does not
apply to SELECT statements
executed within stored routines. It also does not apply to
SELECT statements that do not
produce a result set to be returned to the client. These
include SELECT statements in
subqueries,
CREATE TABLE ...
SELECT, and
INSERT INTO ...
SELECT.
ssl_ca
The path to a file with a list of trusted SSL CAs. This
variable was added in MySQL 5.0.23.
ssl_capath
The path to a directory that contains trusted SSL CA
certificates in PEM format. This variable was added in MySQL
5.0.23.
ssl_cert
The name of the SSL certificate file to use for establishing a
secure connection. This variable was added in MySQL 5.0.23.
ssl_cipher
A list of allowable ciphers to use for SSL encryption. This
variable was added in MySQL 5.0.23.
ssl_key
The name of the SSL key file to use for establishing a secure
connection. This variable was added in MySQL 5.0.23.
storage_engine
The default storage engine (table type). To set the storage
engine at server startup, use the
--default-storage-engine
option. See Section 5.1.2, “Server Command Options”.
sync_frm
If this variable is set to 1, when any nontemporary table is
created its .frm file is synchronized to
disk (using fdatasync()). This is slower
but safer in case of a crash. The default is 1.
system_time_zone
The server system time zone. When the server begins executing,
it inherits a time zone setting from the machine defaults,
possibly modified by the environment of the account used for
running the server or the startup script. The value is used to
set system_time_zone.
Typically the time zone is specified by the
TZ environment variable. It also can be
specified using the
--timezone option of the
mysqld_safe script.
The system_time_zone variable
differs from time_zone.
Although they might have the same value, the latter variable
is used to initialize the time zone for each client that
connects. See Section 9.7, “MySQL Server Time Zone Support”.
table_cache
The number of open tables for all threads. Increasing this
value increases the number of file descriptors that
mysqld requires. You can check whether you
need to increase the table cache by checking the
Opened_tables status
variable. See Section 5.1.6, “Server Status Variables”. If
the value of Opened_tables
is large and you don't do
FLUSH TABLES
often (which just forces all tables to be closed and
reopened), then you should increase the value of the
table_cache variable. For
more information about the table cache, see
Section 7.4.8, “How MySQL Opens and Closes Tables”.
table_lock_wait_timeout
This variable currently is unused.
table_type
This variable is a synonym for
storage_engine. In MySQL
5.0,
storage_engine is the
preferred name.
thread_cache_size
How many threads the server should cache for reuse. When a
client disconnects, the client's threads are put in the cache
if there are fewer than
thread_cache_size threads
there. Requests for threads are satisfied by reusing threads
taken from the cache if possible, and only when the cache is
empty is a new thread created. This variable can be increased
to improve performance if you have a lot of new connections.
(Normally, this doesn't provide a notable performance
improvement if you have a good thread implementation.) By
examining the difference between the
Connections and
Threads_created status
variables, you can see how efficient the thread cache is. For
details, see Section 5.1.6, “Server Status Variables”.
thread_concurrency
This variable is specific to Solaris systems, for which
mysqld invokes the
thr_setconcurrency() with the variable
value. This function enables applications to give the threads
system a hint about the desired number of threads that should
be run at the same time.
thread_stack
The stack size for each thread. Many of the limits detected by
the crash-me test are dependent on this
value. See Section 7.1.3, “The MySQL Benchmark Suite”. The default of
192KB (256KB for 64-bit systems) is large enough for normal
operation. If the thread stack size is too small, it limits
the complexity of the SQL statements that the server can
handle, the recursion depth of stored procedures, and other
memory-consuming actions.
time_format
This variable is unused.
time_zone
The current time zone. This variable is used to initialize the
time zone for each client that connects. By default, the
initial value of this is 'SYSTEM' (which
means, “use the value of
system_time_zone”).
The value can be specified explicitly at server startup with
the --default-time-zone option.
See Section 9.7, “MySQL Server Time Zone Support”.
timed_mutexes
This variable controls whether InnoDB
mutexes are timed. If this variable is set to 0 or
OFF (the default), mutex timing is
disabled. If the variable is set to 1 or
ON, mutex timing is enabled. With timing
enabled, the os_wait_times value in the
output from SHOW
ENGINE INNODB MUTEX indicates the amount of time (in
ms) spent in operating system waits. Otherwise, the value is
0. This variable was added in MySQL 5.0.3.
tmp_table_size
The maximum size of internal in-memory temporary tables. (The
actual limit is determined as the minimum of
tmp_table_size and
max_heap_table_size.) If an
in-memory temporary table exceeds the limit, MySQL
automatically converts it to an on-disk
MyISAM table. Increase the value of
tmp_table_size (and
max_heap_table_size if
necessary) if you do many advanced GROUP BY
queries and you have lots of memory. This variable does not
apply to user-created MEMORY tables.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.5.10, “How MySQL Uses Internal Temporary Tables”.
tmpdir
The directory used for temporary files and temporary tables.
This variable can be set to a list of several paths that are
used in round-robin fashion. Paths should be separated by
colon characters (“:”) on Unix
and semicolon characters (“;”)
on Windows, NetWare, and OS/2.
The multiple-directory feature can be used to spread the load
between several physical disks. If the MySQL server is acting
as a replication slave, you should not set
tmpdir to point to a directory on a
memory-based file system or to a directory that is cleared
when the server host restarts. A replication slave needs some
of its temporary files to survive a machine restart so that it
can replicate temporary tables or
LOAD DATA
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
fails. You can set the slave's temporary directory using the
slave_load_tmpdir variable.
In that case, the slave won't use the general
tmpdir value and you can set
tmpdir to a nonpermanent location.
transaction_alloc_block_size
The amount in bytes by which to increase a per-transaction
memory pool which needs memory. See the description of
transaction_prealloc_size.
transaction_prealloc_size
There is a per-transaction memory pool from which various
transaction-related allocations take memory. The initial size
of the pool in bytes is
transaction_prealloc_size.
For every allocation that cannot be satisfied from the pool
because it has insufficient memory available, the pool is
increased by
transaction_alloc_block_size
bytes. When the transaction ends, the pool is truncated to
transaction_prealloc_size
bytes.
By making
transaction_prealloc_size
sufficiently large to contain all statements within a single
transaction, you can avoid many malloc()
calls.
tx_isolation
The default transaction isolation level. Defaults to
REPEATABLE-READ.
This variable is set by the
SET
TRANSACTION ISOLATION LEVEL statement. See
Section 12.4.6, “SET TRANSACTION Syntax”. If you set
tx_isolation directly to an
isolation level name that contains a space, the name should be
enclosed within quotes, with the space replaced by a dash. For
example:
SET tx_isolation = 'READ-COMMITTED';
Any unique prefix of a valid value may be used to set the
value of this variable.
updatable_views_with_limit
This variable controls whether updates to a view can be made
when the view does not contain all columns of the primary key
defined in the underlying table, if the update statement
contains a LIMIT clause. (Such updates
often are generated by GUI tools.) An update is an
UPDATE or
DELETE statement. Primary key
here means a PRIMARY KEY, or a
UNIQUE index in which no column can contain
NULL.
The variable can have two values:
This variable was added in MySQL 5.0.2.
version
The version number for the server.
Starting with MySQL 5.0.24, the version number will also
indicate whether the server is a standard release (Community)
or Enterprise release (for example,
5.0.28-enterprise-gpl-nt).
version_bdb
The BDB storage engine version.
version_comment
The configure script has a
--with-comment option that allows a comment
to be specified when building MySQL. This variable contains
the value of that comment.
For precompiled binaries, this variable will hold the server
version and license information. Starting with MySQL 5.0.24,
version_comment will include
the full server type and license. For community users this
will appear as MySQL Community Edition - Standard
(GPL). For Enterprise users, the version might be
displayed as MySQL Enterprise Server (GPL).
The corresponding license for your MySQL binary is shown in
parentheses. For server compiled from source, the default
value will be the same as that for Community releases.
version_compile_machine
The type of machine or architecture on which MySQL was built.
version_compile_os
The type of operating system on which MySQL was built.
wait_timeout
The number of seconds the server waits for activity on a
noninteractive connection before closing it. This timeout
applies only to TCP/IP and Unix socket file connections, not
to connections made via named pipes, or shared memory.
On thread startup, the session
wait_timeout value is
initialized from the global
wait_timeout value or from
the global
interactive_timeout value,
depending on the type of client (as defined by the
CLIENT_INTERACTIVE connect option to
mysql_real_connect()). See
also interactive_timeout.
MySQL Enterprise
Expert use of server system variables is part of the service
offered by the MySQL Enterprise Monitor. To subscribe, see
http://www.mysql.com/products/enterprise/advisors.html.
5.1.4. Session System Variables
Several system variables exist only as session variables. These
cannot be set at server startup but can be assigned values at
runtime using the
SET
statement (except for those that are read only). Most of them are
not displayed by SHOW VARIABLES,
but you can obtain their values using
SELECT. This section describes the
session system variables. For information about setting or
displaying their values, see
Section 5.1.5, “Using System Variables”. For example:
mysql> SELECT @@autocommit;
+--------------+
| @@autocommit |
+--------------+
| 1 |
+--------------+
The lettercase of these variables does not matter.
The following table lists the system variables that have only
session scope:
Table 5.3. Session System Variable Summary
autocommit
The autocommit mode. If set to 1, all changes to a table take
effect immediately. If set to 0, you must use
COMMIT to accept a transaction
or ROLLBACK
to cancel it. By default, client connections begin with
autocommit set to 1. If you
change autocommit mode from 0
to 1, MySQL performs an automatic
COMMIT of any open transaction.
Another way to begin a transaction is to use a
START
TRANSACTION or
BEGIN
statement. See Section 12.4.1, “START TRANSACTION,
COMMIT, and
ROLLBACK Syntax”.
big_tables
If set to 1, all temporary tables are stored on disk rather
than in memory. This is a little slower, but the error
The table tbl_name is
full does not occur for
SELECT operations that require
a large temporary table. The default value for a new
connection is 0 (use in-memory temporary tables). Normally,
you should never need to set this variable, because in-memory
tables are automatically converted to disk-based tables as
required.
Note
This variable was formerly named
sql_big_tables.
error_count
The number of errors that resulted from the last statement
that generated messages. This variable is read only. See
Section 12.5.5.14, “SHOW ERRORS Syntax”.
foreign_key_checks
If set to 1 (the default), foreign key constraints for
InnoDB tables are checked. If set to 0,
they are ignored. Disabling foreign key checking can be useful
for reloading InnoDB tables in an order
different from that required by their parent/child
relationships. See
Section 13.2.4.4, “FOREIGN KEY Constraints”.
Setting foreign_key_checks to
0 also affects data definition statements:
DROP DATABASE drops a database
even if it contains tables that have foreign keys that are
referred to by tables outside the database, and
DROP TABLE drops tables that
have foreign keys that are referred to by other tables.
identity
This variable is a synonym for the
last_insert_id variable. It
exists for compatibility with other database systems. You can
read its value with SELECT @@identity, and
set it using SET identity.
insert_id
The value to be used by the following
INSERT or
ALTER TABLE statement when
inserting an AUTO_INCREMENT value. This is
mainly used with the binary log.
last_insert_id
The value to be returned from
LAST_INSERT_ID(). This is
stored in the binary log when you use
LAST_INSERT_ID() in a statement
that updates a table. Setting this variable does not update
the value returned by the
mysql_insert_id() C API
function.
profiling
If set to 0 (the default), statement profiling is disabled. If
set to 1, statement profiling is enabled and the
SHOW PROFILES and
SHOW PROFILE statements provide
access to profiling information. See
Section 12.5.5.29, “SHOW PROFILES Syntax”. This variable was added in
MySQL 5.0.37. Note: This option does not
apply to MySQL Enterprise Server users.
profiling_history_size
The number of statements for which to maintain profiling
information if profiling is
enabled. The default value is 15. The maximum value is 100.
Setting the value to 0 effectively disables profiling. See
Section 12.5.5.29, “SHOW PROFILES Syntax”. This variable was added in
MySQL 5.0.37. Note: This option does not
apply to MySQL Enterprise Server users.
rand_seed1
The rand_seed1 and
rand_seed2 variables exist as
session variables only, and can be set but not read. They are
not shown in the output of SHOW
VARIABLES.
The purpose of these variables is to support replication of
the RAND() function. For
statements that invoke RAND(),
the master passes two values to the slave, where they are used
to seed the random number generator. The slave uses these
values to set the session variables
rand_seed1 and
rand_seed2 so that
RAND() on the slave generates
the same value as on the master.
rand_seed2
See the description for
rand_seed1.
sql_auto_is_null
If this variable is set to 1 (the default), then after a
statement that successfully inserts an automatically generated
AUTO_INCREMENT value, you can find that
value by issuing a statement of the following form:
SELECT * FROM tbl_name WHERE auto_col IS NULL
If the statement returns a row, the value returned is the same
as if you invoked the
LAST_INSERT_ID() function. For
details, including the return value after a multiple-row
insert, see Section 11.10.3, “Information Functions”. If no
AUTO_INCREMENT value was successfully
inserted, the SELECT statement
returns no row.
The behavior of retrieving an
AUTO_INCREMENT value by using an
IS NULL comparison is used by
some ODBC programs, such as Access. See
Section 20.1.7.1.1, “Obtaining Auto-Increment Values”.
This behavior can be disabled by setting
sql_auto_is_null to 0.
sql_big_selects
If set to 0, MySQL aborts
SELECT statements that are
likely to take a very long time to execute (that is,
statements for which the optimizer estimates that the number
of examined rows exceeds the value of
max_join_size). This is
useful when an inadvisable WHERE statement
has been issued. The default value for a new connection is 1,
which allows all SELECT
statements.
If you set the max_join_size
system variable to a value other than
DEFAULT,
sql_big_selects is set to 0.
sql_buffer_result
If set to 1,
sql_buffer_result forces
results from SELECT statements
to be put into temporary tables. This helps MySQL free the
table locks early and can be beneficial in cases where it
takes a long time to send results to the client. The default
value is 0.
sql_log_bin
If set to 0, no logging is done to the binary log for the
client. The client must have the
SUPER privilege to set this
option. The default value is 1.
sql_log_off
If set to 1, no logging is done to the general query log for
this client. The client must have the
SUPER privilege to set this
option. The default value is 0.
sql_log_update
This variable is deprecated, and is mapped to
sql_log_bin.
sql_notes
If set to 1 (the default), warnings of Note
level are recorded. If set to 0, Note
warnings are suppressed. mysqldump includes
output to set this variable to 0 so that reloading the dump
file does not produce warnings for events that do not affect
the integrity of the reload operation.
sql_notes was added in MySQL
5.0.3.
sql_quote_show_create
If set to 1 (the default), the server quotes identifiers for
SHOW CREATE TABLE and
SHOW CREATE DATABASE
statements. If set to 0, quoting is disabled. This option is
enabled by default so that replication works for identifiers
that require quoting. See Section 12.5.5.9, “SHOW CREATE TABLE Syntax”,
and Section 12.5.5.6, “SHOW CREATE DATABASE Syntax”.
sql_safe_updates
If set to 1, MySQL aborts
UPDATE or
DELETE statements that do not
use a key in the WHERE clause or a
LIMIT clause. This makes it possible to
catch UPDATE or
DELETE statements where keys
are not used properly and that would probably change or delete
a large number of rows. The default value is 0.
sql_warnings
This variable controls whether single-row
INSERT statements produce an
information string if warnings occur. The default is 0. Set
the value to 1 to produce an information string.
timestamp =
{timestamp_value |
DEFAULT}
Set the time for this client. This is used to get the original
timestamp if you use the binary log to restore rows.
timestamp_value should be a Unix
epoch timestamp, not a MySQL timestamp.
SET timestamp affects the value returned by
NOW() but not by
SYSDATE(). This means that
timestamp settings in the binary log have no effect on
invocations of SYSDATE(). The
server can be started with the
--sysdate-is-now option to
cause SYSDATE() to be an alias
for NOW(), in which case
SET timestamp affects both functions.
unique_checks
If set to 1 (the default), uniqueness checks for secondary
indexes in InnoDB tables are performed. If
set to 0, storage engines are allowed to assume that duplicate
keys are not present in input data. If you know for certain
that your data does not contain uniqueness violations, you can
set this to 0 to speed up large table imports to
InnoDB.
Note that setting this variable to 0 does not
require storage engines to ignore
duplicate keys. An engine is still allowed to check for them
and issue duplicate-key errors if it detects them.
warning_count
The number of errors, warnings, and notes that resulted from
the last statement that generated messages. This variable is
read only. See Section 12.5.5.37, “SHOW WARNINGS Syntax”.
5.1.5. Using System Variables
The MySQL server maintains many system variables that indicate how
it is configured. Section 5.1.3, “Server System Variables”,
describes the meaning of these variables. Each system variable has
a default value. System variables can be set at server startup
using options on the command line or in an option file. Most of
them can be changed dynamically while the server is running by
means of the
SET
statement, which enables you to modify operation of the server
without having to stop and restart it. You can refer to system
variable values in expressions.
The server maintains two kinds of system variables. Global
variables affect the overall operation of the server. Session
variables affect its operation for individual client connections.
A given system variable can have both a global and a session
value. Global and session system variables are related as follows:
When the server starts, it initializes all global variables to
their default values. These defaults can be changed by options
specified on the command line or in an option file. (See
Section 4.2.3, “Specifying Program Options”.)
The server also maintains a set of session variables for each
client that connects. The client's session variables are
initialized at connect time using the current values of the
corresponding global variables. For example, the client's SQL
mode is controlled by the session
sql_mode value, which is
initialized when the client connects to the value of the
global sql_mode value.
System variable values can be set globally at server startup by
using options on the command line or in an option file. When you
use a startup option to set a variable that takes a numeric value,
the value can be given with a suffix of K,
M, or G (either uppercase or
lowercase) to indicate a multiplier of 1024,
10242 or
10243; that is, units of kilobytes,
megabytes, or gigabytes, respectively. Thus, the following command
starts the server with a query cache size of 16 megabytes and a
maximum packet size of one gigabyte:
mysqld --query_cache_size=16M --max_allowed_packet=1G
Within an option file, those variables are set like this:
[mysqld]
query_cache_size=16M
max_allowed_packet=1G
The lettercase of suffix letters does not matter;
16M and 16m are equivalent,
as are 1G and 1g.
If you want to restrict the maximum value to which a system
variable can be set at runtime with the
SET
statement, you can specify this maximum by using an option of the
form
--maximum-var_name=value
at server startup. For example, to prevent the value of
query_cache_size from being
increased to more than 32MB at runtime, use the option
--maximum-query_cache_size=32M.
Many system variables are dynamic and can be changed while the
server runs by using the
SET
statement. For a list, see
Section 5.1.5.2, “Dynamic System Variables”. To change a system
variable with
SET, refer
to it as var_name, optionally preceded
by a modifier:
To indicate explicitly that a variable is a global variable,
precede its name by GLOBAL or
@@global.. The
SUPER privilege is required to
set global variables.
To indicate explicitly that a variable is a session variable,
precede its name by SESSION,
@@session., or @@.
Setting a session variable requires no special privilege, but
a client can change only its own session variables, not those
of any other client.
LOCAL and @@local. are
synonyms for SESSION and
@@session..
If no modifier is present,
SET
changes the session variable.
A SET
statement can contain multiple variable assignments, separated by
commas. If you set several system variables, the most recent
GLOBAL or SESSION modifier
in the statement is used for following variables that have no
modifier specified.
Examples:
SET sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;
The @@var_name
syntax for system variables is supported for compatibility with
some other database systems.
If you change a session system variable, the value remains in
effect until your session ends or until you change the variable to
a different value. The change is not visible to other clients.
If you change a global system variable, the value is remembered
and used for new connections until the server restarts. (To make a
global system variable setting permanent, you should set it in an
option file.) The change is visible to any client that accesses
that global variable. However, the change affects the
corresponding session variable only for clients that connect after
the change. The global variable change does not affect the session
variable for any client that is currently connected (not even that
of the client that issues the
SET GLOBAL
statement).
To prevent incorrect usage, MySQL produces an error if you use
SET GLOBAL
with a variable that can only be used with
SET SESSION
or if you do not specify GLOBAL (or
@@global.) when setting a global variable.
To set a SESSION variable to the
GLOBAL value or a GLOBAL
value to the compiled-in MySQL default value, use the
DEFAULT keyword. For example, the following two
statements are identical in setting the session value of
max_join_size to the global
value:
SET max_join_size=DEFAULT;
SET @@session.max_join_size=@@global.max_join_size;
Not all system variables can be set to DEFAULT.
In such cases, use of DEFAULT results in an
error.
You can refer to the values of specific global or sesson system
variables in expressions by using one of the
@@-modifiers. For example, you can retrieve
values in a SELECT statement like
this:
SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;
When you refer to a system variable in an expression as
@@var_name (that is,
when you do not specify @@global. or
@@session.), MySQL returns the session value if
it exists and the global value otherwise. (This differs from
SET @@var_name =
value, which always refers to
the session value.)
Note
Some variables displayed by SHOW VARIABLES
may not be available using SELECT
@@var_name syntax; an
Unknown system variable occurs. As a
workaround in such cases, you can use SHOW VARIABLES
LIKE 'var_name'.
Suffixes for specifying a value multiplier can be used when
setting a variable at server startup, but not to set the value
with SET at
runtime. On the other hand, with
SET you can
assign a variable's value using an expression, which is not true
when you set a variable at server startup. For example, the first
of the following lines is legal at server startup, but the second
is not:
shell> mysql --max_allowed_packet=16M
shell> mysql --max_allowed_packet=16*1024*1024
Conversely, the second of the following lines is legal at runtime,
but the first is not:
mysql> SET GLOBAL max_allowed_packet=16M;
mysql> SET GLOBAL max_allowed_packet=16*1024*1024;
Note
Some system variables can be enabled with the
SET
statement by setting them to ON or
1, or disabled by setting them to
OFF or 0. However, to set
such a variable on the command line or in an option file, you
must set it to 1 or 0;
setting it to ON or OFF
will not work. For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
To display system variable names and values, use the
SHOW VARIABLES statement:
mysql> SHOW VARIABLES;
+--------+--------------------------------------------------------------+
| Variable_name | Value |
+--------+--------------------------------------------------------------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
| automatic_sp_privileges | ON |
| back_log | 50 |
| basedir | / |
| bdb_cache_size | 8388600 |
| bdb_home | /var/lib/mysql/ |
| bdb_log_buffer_size | 32768 |
| bdb_logdir | |
| bdb_max_lock | 10000 |
| bdb_shared_data | OFF |
| bdb_tmpdir | /tmp/ |
| binlog_cache_size | 32768 |
| bulk_insert_buffer_size | 8388608 |
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
| collation_connection | latin1_swedish_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
...
| innodb_additional_mem_pool_size | 1048576 |
| innodb_autoextend_increment | 8 |
| innodb_buffer_pool_awe_mem_mb | 0 |
| innodb_buffer_pool_size | 8388608 |
| innodb_checksums | ON |
| innodb_commit_concurrency | 0 |
| innodb_concurrency_tickets | 500 |
| innodb_data_file_path | ibdata1:10M:autoextend |
| innodb_data_home_dir | |
...
| version | 5.0.19 |
| version_comment | MySQL Community Edition - (GPL) |
| version_compile_machine | i686 |
| version_compile_os | pc-linux-gnu |
| wait_timeout | 28800 |
+--------+--------------------------------------------------------------+
With a LIKE clause, the statement
displays only those variables that match the pattern. To obtain a
specific variable name, use a LIKE
clause as shown:
SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';
To get a list of variables whose name match a pattern, use the
“%” wildcard character in a
LIKE clause:
SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';
Wildcard characters can be used in any position within the pattern
to be matched. Strictly speaking, because
“_” is a wildcard that matches any
single character, you should escape it as
“\_” to match it literally. In
practice, this is rarely necessary.
For SHOW VARIABLES, if you specify
neither GLOBAL nor SESSION,
MySQL returns SESSION values.
The reason for requiring the GLOBAL keyword
when setting GLOBAL-only variables but not when
retrieving them is to prevent problems in the future. If we were
to remove a SESSION variable that has the same
name as a GLOBAL variable, a client with the
SUPER privilege might accidentally
change the GLOBAL variable rather than just the
SESSION variable for its own connection. If we
add a SESSION variable with the same name as a
GLOBAL variable, a client that intends to
change the GLOBAL variable might find only its
own SESSION variable changed.
5.1.5.1. Structured System Variables
A structured variable differs from a regular system variable in
two respects:
Its value is a structure with components that specify server
parameters considered to be closely related.
There might be several instances of a given type of
structured variable. Each one has a different name and
refers to a different resource maintained by the server.
MySQL 5.0 supports one structured variable type,
which specifies parameters governing the operation of key
caches. A key cache structured variable has these components:
This section describes the syntax for referring to structured
variables. Key cache variables are used for syntax examples, but
specific details about how key caches operate are found
elsewhere, in Section 7.4.5, “The MyISAM Key Cache”.
To refer to a component of a structured variable instance, you
can use a compound name in
instance_name.component_name format.
Examples:
hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size
For each structured system variable, an instance with the name
of default is always predefined. If you refer
to a component of a structured variable without any instance
name, the default instance is used. Thus,
default.key_buffer_size and
key_buffer_size both refer to
the same system variable.
Structured variable instances and components follow these naming
rules:
For a given type of structured variable, each instance must
have a name that is unique within
variables of that type. However, instance names need not be
unique across structured variable
types. For example, each structured variable has an instance
named default, so
default is not unique across variable
types.
The names of the components of each structured variable type
must be unique across all system variable names. If this
were not true (that is, if two different types of structured
variables could share component member names), it would not
be clear which default structured variable to use for
references to member names that are not qualified by an
instance name.
If a structured variable instance name is not legal as an
unquoted identifier, refer to it as a quoted identifier
using backticks. For example, hot-cache
is not legal, but `hot-cache` is.
global, session, and
local are not legal instance names. This
avoids a conflict with notation such as
@@global.var_name
for referring to nonstructured system variables.
Currently, the first two rules have no possibility of being
violated because the only structured variable type is the one
for key caches. These rules will assume greater significance if
some other type of structured variable is created in the future.
With one exception, you can refer to structured variable
components using compound names in any context where simple
variable names can occur. For example, you can assign a value to
a structured variable using a command-line option:
shell> mysqld --hot_cache.key_buffer_size=64K
In an option file, use this syntax:
[mysqld]
hot_cache.key_buffer_size=64K
If you start the server with this option, it creates a key cache
named hot_cache with a size of 64KB in
addition to the default key cache that has a default size of
8MB.
Suppose that you start the server as follows:
shell> mysqld --key_buffer_size=256K \
--extra_cache.key_buffer_size=128K \
--extra_cache.key_cache_block_size=2048
In this case, the server sets the size of the default key cache
to 256KB. (You could also have written
--default.key_buffer_size=256K.) In addition,
the server creates a second key cache named
extra_cache that has a size of 128KB, with
the size of block buffers for caching table index blocks set to
2048 bytes.
The following example starts the server with three different key
caches having sizes in a 3:1:1 ratio:
shell> mysqld --key_buffer_size=6M \
--hot_cache.key_buffer_size=2M \
--cold_cache.key_buffer_size=2M
Structured variable values may be set and retrieved at runtime
as well. For example, to set a key cache named
hot_cache to a size of 10MB, use either of
these statements:
mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@global.hot_cache.key_buffer_size = 10*1024*1024;
To retrieve the cache size, do this:
mysql> SELECT @@global.hot_cache.key_buffer_size;
However, the following statement does not work. The variable is
not interpreted as a compound name, but as a simple string for a
LIKE pattern-matching operation:
mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';
This is the exception to being able to use structured variable
names anywhere a simple variable name may occur.
5.1.5.2. Dynamic System Variables
Many server system variables are dynamic and can be set at
runtime using SET
GLOBAL or
SET
SESSION. You can also obtain their values using
SELECT. See
Section 5.1.5, “Using System Variables”.
The following table shows the full list of all dynamic system
variables. The last column indicates for each variable whether
GLOBAL or SESSION (or
both) apply. The table also lists session options that can be
set with the
SET
statement. Section 5.1.4, “Session System Variables”, discusses
these options.
Variables that have a type of “string” take a
string value. Variables that have a type of
“numeric” take a numeric value. Variables that have
a type of “boolean” can be set to 0, 1,
ON or OFF. (If you set
them on the command line or in an option file, use the numeric
values.) Variables that are marked as “enumeration”
normally should be set to one of the available values for the
variable, but can also be set to the number that corresponds to
the desired enumeration value. For enumerated system variables,
the first enumeration value corresponds to 0. This differs from
ENUM columns, for which the first
enumeration value corresponds to 1.
Table 5.4. Dynamic Variable Summary MySQL Enterprise
Improper configuration of system variables can adversely
affect performance and security. The MySQL Enterprise Monitor
continually monitors system variables and provides expert
advice about appropriate settings. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
5.1.6. Server Status Variables
The server maintains many status variables that provide
information about its operation. You can view these variables and
their values by using the SHOW [GLOBAL | SESSION]
STATUS statement (see Section 12.5.5.32, “SHOW STATUS Syntax”).
The optional GLOBAL keyword aggregates the
values over all connections, and SESSION shows
the values for the current connection.
mysql> SHOW GLOBAL STATUS;
+-----------------------------------+------------+
| Variable_name | Value |
+-----------------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
| Bytes_sent | 1176560426 |
...
| Connections | 30023 |
| Created_tmp_disk_tables | 0 |
| Created_tmp_files | 3 |
| Created_tmp_tables | 2 |
...
| Threads_created | 217 |
| Threads_running | 88 |
| Uptime | 1389872 |
+-----------------------------------+------------+
The following table lists all available server status variables:
Table 5.5. Status Variable Summary Note
Before MySQL 5.0.2, SHOW STATUS
returned global status values. Because the default as of 5.0.2
is to return session values, this is incompatible with previous
versions. To issue a SHOW STATUS
statement that will retrieve global status values for all
versions of MySQL, write it like this:
SHOW /*!50002 GLOBAL */ STATUS;
Many status variables are reset to 0 by the FLUSH
STATUS statement.
MySQL Enterprise
For expert advice on using status variables, subscribe to the
MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
The status variables have the following meanings. Variables with
no version indicated were already present prior to MySQL
5.0. For information regarding their implementation
history, see MySQL 3.23, 4.0, 4.1 Reference Manual.
For meanings of status variables specific to MySQL Cluster, see
Section 17.3.4.4, “MySQL Cluster Status Variables”.
Aborted_clients
The number of connections that were aborted because the client
died without closing the connection properly. See
Section B.5.2.11, “Communication Errors and Aborted Connections”.
Aborted_connects
The number of failed attempts to connect to the MySQL server.
See Section B.5.2.11, “Communication Errors and Aborted Connections”.
Binlog_cache_disk_use
The number of transactions that used the temporary binary log
cache but that exceeded the value of
binlog_cache_size and used a
temporary file to store statements from the transaction.
Binlog_cache_use
The number of transactions that used the temporary binary log
cache.
Bytes_received
The number of bytes received from all clients.
Bytes_sent
The number of bytes sent to all clients.
Com_xxx
The Com_xxx
statement counter variables indicate the number of times each
xxx statement has been executed.
There is one status variable for each type of statement. For
example, Com_delete and
Com_insert count
DELETE and
INSERT statements,
respectively. However, if a query result is returned from
query cache, the server increments the
Qcache_hits status variable,
not Com_select. See
Section 7.5.5.4, “Query Cache Status and Maintenance”.
All of the
Com_stmt_xxx
variables are increased even if a prepared statement argument
is unknown or an error occurred during execution. In other
words, their values correspond to the number of requests
issued, not to the number of requests successfully completed.
The Com_stmt_xxx
status variables were added in 5.0.8:
Com_stmt_prepare
Com_stmt_execute
Com_stmt_fetch
Com_stmt_send_long_data
Com_stmt_reset
Com_stmt_close
Those variables stand for prepared statement commands. Their
names refer to the
COM_xxx command
set used in the network layer. In other words, their values
increase whenever prepared statement API calls such as
mysql_stmt_prepare(),
mysql_stmt_execute(), and so forth are
executed. However, Com_stmt_prepare,
Com_stmt_execute and
Com_stmt_close also increase for
PREPARE,
EXECUTE, or
DEALLOCATE PREPARE,
respectively. Additionally, the values of the older statement
counter variables Com_prepare_sql,
Com_execute_sql, and
Com_dealloc_sql increase for the
PREPARE,
EXECUTE, and
DEALLOCATE PREPARE statements.
Com_stmt_fetch stands for the total number
of network round-trips issued when fetching from cursors.
Compression
Whether the client connection uses compression in the
client/server protocol. Added in MySQL 5.0.16.
Connections
The number of connection attempts (successful or not) to the
MySQL server.
Created_tmp_disk_tables
The number of internal on-disk temporary tables created by the
server while executing statements.
If an internal temporary table is created initially as an
in-memory table but becomes too large, MySQL automatically
converts it to an on-disk table. The maximum size for
in-memory temporary tables is the minimum of the
tmp_table_size and
max_heap_table_size values.
If Created_tmp_disk_tables
is large, you may want to increase the
tmp_table_size or
max_heap_table_size values.
value to lessen the likelihood that internal temporary tables
in memory will be converted to on-disk tables.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.5.10, “How MySQL Uses Internal Temporary Tables”.
Created_tmp_files
How many temporary files mysqld has
created.
Created_tmp_tables
The number of internal temporary tables created by the server
while executing statements.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.5.10, “How MySQL Uses Internal Temporary Tables”.
Delayed_errors
The number of rows written with INSERT
DELAYED for which some error occurred (probably
duplicate key).
Delayed_insert_threads
The number of INSERT DELAYED
handler threads in use.
Delayed_writes
The number of INSERT DELAYED
rows written.
Flush_commands
The number of executed FLUSH
statements.
Handler_commit
The number of internal COMMIT
statements.
Handler_delete
The number of times that rows have been deleted from tables.
Handler_prepare
A counter for the prepare phase of two-phase commit
operations. Added in MySQL 5.0.3.
Handler_read_first
The number of times the first entry in an index was read. If
this value is high, it suggests that the server is doing a lot
of full index scans; for example, SELECT col1 FROM
foo, assuming that col1 is
indexed.
Handler_read_key
The number of requests to read a row based on a key. If this
value is high, it is a good indication that your tables are
properly indexed for your queries.
Handler_read_next
The number of requests to read the next row in key order. This
value is incremented if you are querying an index column with
a range constraint or if you are doing an index scan.
Handler_read_prev
The number of requests to read the previous row in key order.
This read method is mainly used to optimize ORDER BY
... DESC.
Handler_read_rnd
The number of requests to read a row based on a fixed
position. This value is high if you are doing a lot of queries
that require sorting of the result. You probably have a lot of
queries that require MySQL to scan entire tables or you have
joins that don't use keys properly.
Handler_read_rnd_next
The number of requests to read the next row in the data file.
This value is high if you are doing a lot of table scans.
Generally this suggests that your tables are not properly
indexed or that your queries are not written to take advantage
of the indexes you have.
Handler_rollback
The number of requests for a storage engine to perform a
rollback operation.
Handler_savepoint
The number of requests for a storage engine to place a
savepoint. Added in MySQL 5.0.3.
Handler_savepoint_rollback
The number of requests for a storage engine to roll back to a
savepoint. Added in MySQL 5.0.3.
Handler_update
The number of requests to update a row in a table.
Handler_write
The number of requests to insert a row in a table.
Innodb_buffer_pool_pages_data
The number of pages containing data (dirty or clean). Added in
MySQL 5.0.2.
Innodb_buffer_pool_pages_dirty
The number of pages currently dirty. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_flushed
The number of buffer pool page-flush requests. Added in MySQL
5.0.2.
Innodb_buffer_pool_pages_free
The number of free pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_latched
The number of latched pages in InnoDB
buffer pool. These are pages currently being read or written
or that cannot be flushed or removed for some other reason.
Added in MySQL 5.0.2. Calculation of this variable is
expensive, so as of MySQL 5.0.68, it is available only when
the UNIV_DEBUG system is defined at server
build time.
Innodb_buffer_pool_pages_misc
The number of pages that are busy because they have been
allocated for administrative overhead such as row locks or the
adaptive hash index. This value can also be calculated as
Innodb_buffer_pool_pages_total
–
Innodb_buffer_pool_pages_free
–
Innodb_buffer_pool_pages_data.
Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_total
The total size of buffer pool, in pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_rnd
The number of “random” read-aheads initiated by
InnoDB. This happens when a query scans a
large portion of a table but in random order. Added in MySQL
5.0.2.
Innodb_buffer_pool_read_ahead_seq
The number of sequential read-aheads initiated by
InnoDB. This happens when
InnoDB does a sequential full table scan.
Added in MySQL 5.0.2.
Innodb_buffer_pool_read_requests
The number of logical read requests InnoDB
has done. Added in MySQL 5.0.2.
Innodb_buffer_pool_reads
The number of logical reads that InnoDB
could not satisfy from the buffer pool, and had to read
directly from the disk.
Innodb_buffer_pool_wait_free
Normally, writes to the InnoDB buffer pool
happen in the background. However, if it is necessary to read
or create a page and no clean pages are available, it is also
necessary to wait for pages to be flushed first. This counter
counts instances of these waits. If the buffer pool size has
been set properly, this value should be small. Added in MySQL
5.0.2.
Innodb_buffer_pool_write_requests
The number writes done to the InnoDB buffer
pool. Added in MySQL 5.0.2.
Innodb_data_fsyncs
The number of fsync() operations so far.
Added in MySQL 5.0.2.
Innodb_data_pending_fsyncs
The current number of pending fsync()
operations. Added in MySQL 5.0.2.
Innodb_data_pending_reads
The current number of pending reads. Added in MySQL 5.0.2.
Innodb_data_pending_writes
The current number of pending writes. Added in MySQL 5.0.2.
Innodb_data_read
The amount of data read since the server was started. Added in
MySQL 5.0.2.
Innodb_data_reads
The total number of data reads. Added in MySQL 5.0.2.
Innodb_data_writes
The total number of data writes. Added in MySQL 5.0.2.
Innodb_data_written
The amount of data written so far, in bytes. Added in MySQL
5.0.2.
Innodb_dblwr_pages_written
The number of pages that have been written for doublewrite
operations. Added in MySQL 5.0.2. See
Section 13.2.11.1, “InnoDB Disk I/O”.
Innodb_dblwr_writes
The number of doublewrite operations that have been performed.
Added in MySQL 5.0.2. See Section 13.2.11.1, “InnoDB Disk I/O”.
Innodb_log_waits
The number of times that the log buffer was too small and a
wait was required for it to be flushed before continuing.
Added in MySQL 5.0.2.
Innodb_log_write_requests
The number of log write requests. Added in MySQL 5.0.2.
Innodb_log_writes
The number of physical writes to the log file. Added in MySQL
5.0.2.
Innodb_os_log_fsyncs
The number of fsync() writes done to the
log file. Added in MySQL 5.0.2.
Innodb_os_log_pending_fsyncs
The number of pending log file fsync()
operations. Added in MySQL 5.0.2.
Innodb_os_log_pending_writes
The number of pending log file writes. Added in MySQL 5.0.2.
Innodb_os_log_written
The number of bytes written to the log file. Added in MySQL
5.0.2.
Innodb_page_size
The compiled-in InnoDB page size (default
16KB). Many values are counted in pages; the page size allows
them to be easily converted to bytes. Added in MySQL 5.0.2.
Innodb_pages_created
The number of pages created. Added in MySQL 5.0.2.
Innodb_pages_read
The number of pages read. Added in MySQL 5.0.2.
Innodb_pages_written
The number of pages written. Added in MySQL 5.0.2.
Innodb_row_lock_current_waits
The number of row locks currently being waited for. Added in
MySQL 5.0.3.
Innodb_row_lock_time
The total time spent in acquiring row locks, in milliseconds.
Added in MySQL 5.0.3.
Innodb_row_lock_time_avg
The average time to acquire a row lock, in milliseconds. Added
in MySQL 5.0.3.
Innodb_row_lock_time_max
The maximum time to acquire a row lock, in milliseconds. Added
in MySQL 5.0.3.
Innodb_row_lock_waits
The number of times a row lock had to be waited for. Added in
MySQL 5.0.3.
Innodb_rows_deleted
The number of rows deleted from InnoDB
tables. Added in MySQL 5.0.2.
Innodb_rows_inserted
The number of rows inserted into InnoDB
tables. Added in MySQL 5.0.2.
Innodb_rows_read
The number of rows read from InnoDB tables.
Added in MySQL 5.0.2.
Innodb_rows_updated
The number of rows updated in InnoDB
tables. Added in MySQL 5.0.2.
Key_blocks_not_flushed
The number of key blocks in the key cache that have changed
but have not yet been flushed to disk.
Key_blocks_unused
The number of unused blocks in the key cache. You can use this
value to determine how much of the key cache is in use; see
the discussion of
key_buffer_size in
Section 5.1.3, “Server System Variables”.
Key_blocks_used
The number of used blocks in the key cache. This value is a
high-water mark that indicates the maximum number of blocks
that have ever been in use at one time.
Key_read_requests
The number of requests to read a key block from the cache.
Key_reads
The number of physical reads of a key block from disk. If
Key_reads is large, then
your key_buffer_size value is
probably too small. The cache miss rate can be calculated as
Key_reads/Key_read_requests.
Key_write_requests
The number of requests to write a key block to the cache.
Key_writes
The number of physical writes of a key block to disk.
Last_query_cost
The total cost of the last compiled query as computed by the
query optimizer. This is useful for comparing the cost of
different query plans for the same query. The default value of
0 means that no query has been compiled yet. This variable was
added in MySQL 5.0.1, with a default value of -1. In MySQL
5.0.7, the default was changed to 0; also in version 5.0.7,
the scope of Last_query_cost
was changed to session rather than global.
The Last_query_cost value
can be computed accurately only for simple “flat”
queries, not complex queries such as those with subqueries or
UNION. For the latter, the
value is set to 0.
Prior to MySQL 5.0.16, this variable was not updated for
queries served from the query cache.
Max_used_connections
The maximum number of connections that have been in use
simultaneously since the server started.
Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT
DELAY queues.
Open_files
The number of files that are open. This count includes regular
files opened by the server. It does not include other types of
files such as sockets or pipes. Also, the count does not
include files that storage engines open using their own
internal functions rather than asking the server level to do
so.
Open_streams
The number of streams that are open (used mainly for logging).
Open_tables
The number of tables that are open.
Opened_tables
The number of tables that have been opened. If
Opened_tables is big, your
table_cache value is probably
too small.
Prepared_stmt_count
The current number of prepared statements. (The maximum number
of statements is given by the
max_prepared_stmt_count
system variable.) This variable was added in MySQL 5.0.32.
Qcache_free_blocks
The number of free memory blocks in the query cache.
Qcache_free_memory
The amount of free memory for the query cache.
Qcache_hits
The number of query cache hits.
Qcache_inserts
The number of queries added to the query cache.
Qcache_lowmem_prunes
The number of queries that were deleted from the query cache
because of low memory.
Qcache_not_cached
The number of noncached queries (not cacheable, or not cached
due to the query_cache_type
setting).
Qcache_queries_in_cache
The number of queries registered in the query cache.
Qcache_total_blocks
The total number of blocks in the query cache.
Queries
The number of statements executed by the server. This variable
includes statements executed within stored programs, unlike
the Questions variable. This variable was
added in MySQL 5.0.76.
Questions
The number of statements executed by the server. As of MySQL
5.0.72, this includes only statements sent to the server by
clients and no longer includes statements executed within
stored programs, unlike the Queries
variable.
Rpl_status
The status of fail-safe replication (not yet implemented).
Select_full_join
The number of joins that perform table scans because they do
not use indexes. If this value is not 0, you should carefully
check the indexes of your tables.
Select_full_range_join
The number of joins that used a range search on a reference
table.
Select_range
The number of joins that used ranges on the first table. This
is normally not a critical issue even if the value is quite
large.
Select_range_check
The number of joins without keys that check for key usage
after each row. If this is not 0, you should carefully check
the indexes of your tables.
Select_scan
The number of joins that did a full scan of the first table.
Slave_open_temp_tables
The number of temporary tables that the slave SQL thread
currently has open.
Slave_retried_transactions
The total number of times since startup that the replication
slave SQL thread has retried transactions. This variable was
added in version 5.0.4.
Slave_running
This is ON if this server is a replication
slave that is connected to a replication master, and both the
I/O and SQL threads are running; otherwise, it is
OFF.
Slow_launch_threads
The number of threads that have taken more than
slow_launch_time seconds to
create.
Slow_queries
The number of queries that have taken more than
long_query_time seconds. See
Section 5.2.4, “The Slow Query Log”.
Sort_merge_passes
The number of merge passes that the sort algorithm has had to
do. If this value is large, you should consider increasing the
value of the sort_buffer_size
system variable.
Sort_range
The number of sorts that were done using ranges.
Sort_rows
The number of sorted rows.
Sort_scan
The number of sorts that were done by scanning the table.
Ssl_accept_renegotiates
The number of negotiates needed to establish the connection.
Ssl_accepts
The number of accepted SSL connections.
Ssl_callback_cache_hits
The number of callback cache hits.
Ssl_cipher
The current SSL cipher (empty for non-SSL connections).
Ssl_cipher_list
The list of possible SSL ciphers.
Ssl_client_connects
The number of SSL connection attempts to an SSL-enabled
master.
Ssl_connect_renegotiates
The number of negotiates needed to establish the connection to
an SSL-enabled master.
Ssl_ctx_verify_depth
The SSL context verification depth (how many certificates in
the chain are tested).
Ssl_ctx_verify_mode
The SSL context verification mode.
Ssl_default_timeout
The default SSL timeout.
Ssl_finished_accepts
The number of successful SSL connections to the server.
Ssl_finished_connects
The number of successful slave connections to an SSL-enabled
master.
Ssl_session_cache_hits
The number of SSL session cache hits.
Ssl_session_cache_misses
The number of SSL session cache misses.
Ssl_session_cache_mode
The SSL session cache mode.
Ssl_session_cache_overflows
The number of SSL session cache overflows.
Ssl_session_cache_size
The SSL session cache size.
Ssl_session_cache_timeouts
The number of SSL session cache timeouts.
Ssl_sessions_reused
How many SSL connections were reused from the cache.
Ssl_used_session_cache_entries
How many SSL session cache entries were used.
Ssl_verify_depth
The verification depth for replication SSL connections.
Ssl_verify_mode
The verification mode for replication SSL connections.
Ssl_version
The SSL version number.
Table_locks_immediate
The number of times that a request for a table lock could be
granted immediately.
Table_locks_waited
The number of times that a request for a table lock could not
be granted immediately and a wait was needed. If this is high
and you have performance problems, you should first optimize
your queries, and then either split your table or tables or
use replication.
Tc_log_max_pages_used
For the memory-mapped implementation of the log that is used
by mysqld when it acts as the transaction
coordinator for recovery of internal XA transactions, this
variable indicates the largest number of pages used for the
log since the server started. If the product of
Tc_log_max_pages_used and
Tc_log_page_size is always
significantly less than the log size, the size is larger than
necessary and can be reduced. (The size is set by the
--log-tc-size option.
Currently, this variable is unused: It is unneeded for binary
log-based recovery, and the memory-mapped recovery log method
is not used unless the number of storage engines capable of
two-phase commit is greater than one.
(InnoDB is the only applicable engine.)
Added in MySQL 5.0.3.
Tc_log_page_size
The page size used for the memory-mapped implementation of the
XA recovery log. The default value is determined using
getpagesize(). Currently, this variable is
unused for the same reasons as described for
Tc_log_max_pages_used. Added
in MySQL 5.0.3.
Tc_log_page_waits
For the memory-mapped implementation of the recovery log, this
variable increments each time the server was not able to
commit a transaction and had to wait for a free page in the
log. If this value is large, you might want to increase the
log size (with the
--log-tc-size option). For
binary log-based recovery, this variable increments each time
the binary log cannot be closed because there are two-phase
commits in progress. (The close operation waits until all such
transactions are finished.) Added in MySQL 5.0.3.
Threads_cached
The number of threads in the thread cache.
Threads_connected
The number of currently open connections.
Threads_created
The number of threads created to handle connections. If
Threads_created is big, you
may want to increase the
thread_cache_size value. The
cache miss rate can be calculated as
Threads_created/Connections.
Threads_running
The number of threads that are not sleeping.
Uptime
The number of seconds that the server has been up.
Uptime_since_flush_status
The number of seconds since the most recent FLUSH
STATUS statement. This variable was added in 5.0.35.
(MySQL Community only)
The MySQL server can operate in different SQL modes, and can apply
these modes differently for different clients. This capability
enables each application to tailor the server's operating mode to
its own requirements.
For answers to some questions that are often asked about server
SQL modes in MySQL, see Section A.3, “MySQL 5.0 FAQ — Server SQL Mode”.
Modes define what SQL syntax MySQL should support and what kind of
data validation checks it should perform. This makes it easier to
use MySQL in different environments and to use MySQL together with
other database servers.
You can set the default SQL mode by starting
mysqld with the
--sql-mode="modes"
option, or by using
sql-mode="modes"
in my.cnf (Unix operating systems) or
my.ini (Windows).
modes is a list of different modes
separated by comma (“,”)
characters. The default value is empty (no modes set). The
modes value also can be empty
(--sql-mode="" on the command line,
or sql-mode="" in
my.cnf on Unix systems or in
my.ini on Windows) if you want to clear it
explicitly.
You can change the SQL mode at runtime by using a SET
[GLOBAL|SESSION]
sql_mode='modes' statement to
set the sql_mode system value.
Setting the GLOBAL variable requires the
SUPER privilege and affects the
operation of all clients that connect from that time on. Setting
the SESSION variable affects only the current
client. Any client can change its own session
sql_mode value at any time.
You can retrieve the current global or session
sql_mode value with the following
statements:
SELECT @@GLOBAL.sql_mode;
SELECT @@SESSION.sql_mode;
The most important sql_mode
values are probably these:
ANSI
This mode changes syntax and behavior to conform more closely
to standard SQL.
STRICT_TRANS_TABLES
If a value could not be inserted as given into a transactional
table, abort the statement. For a nontransactional table,
abort the statement if the value occurs in a single-row
statement or the first row of a multiple-row statement. More
detail is given later in this section. (Implemented in MySQL
5.0.2)
TRADITIONAL
Make MySQL behave like a “traditional” SQL
database system. A simple description of this mode is
“give an error instead of a warning” when
inserting an incorrect value into a column.
Note
The
INSERT/UPDATE
aborts as soon as the error is noticed. This may not be what
you want if you are using a nontransactional storage engine,
because data changes made prior to the error may not be
rolled back, resulting in a “partially done”
update. (Added in MySQL 5.0.2)
When this manual refers to “strict mode,” it means a
mode where at least one of
STRICT_TRANS_TABLES or
STRICT_ALL_TABLES is enabled.
The following list describes all supported modes:
ALLOW_INVALID_DATES
Don't do full checking of dates. Check only that the month is
in the range from 1 to 12 and the day is in the range from 1
to 31. This is very convenient for Web applications where you
obtain year, month, and day in three different fields and you
want to store exactly what the user inserted (without date
validation). This mode applies to
DATE and
DATETIME columns. It does not
apply TIMESTAMP columns, which
always require a valid date.
This mode is implemented in MySQL 5.0.2. Before 5.0.2, this
was the default MySQL date-handling mode. As of 5.0.2, the
server requires that month and day values be legal, and not
merely in the range 1 to 12 and 1 to 31, respectively. With
strict mode disabled, invalid dates such as
'2004-04-31' are converted to
'0000-00-00' and a warning is generated.
With strict mode enabled, invalid dates generate an error. To
allow such dates, enable
ALLOW_INVALID_DATES.
ANSI_QUOTES
Treat “"” as an identifier
quote character (like the “`”
quote character) and not as a string quote character. You can
still use “`” to quote
identifiers with this mode enabled. With
ANSI_QUOTES enabled, you
cannot use double quotes to quote literal strings, because it
is interpreted as an identifier.
ERROR_FOR_DIVISION_BY_ZERO
Produce an error in strict mode (otherwise a warning) when a
division by zero (or MOD(X,0))
occurs during an INSERT or
UPDATE. If this mode is not
enabled, MySQL instead returns NULL for
divisions by zero. For
INSERT
IGNORE or UPDATE IGNORE, MySQL
generates a warning for divisions by zero, but the result of
the operation is NULL. (Implemented in
MySQL 5.0.2)
HIGH_NOT_PRECEDENCE
From MySQL 5.0.2 on, the precedence of the
NOT operator is such that
expressions such as NOT a BETWEEN b AND c
are parsed as NOT (a BETWEEN b AND c).
Before MySQL 5.0.2, the expression is parsed as (NOT
a) BETWEEN b AND c. The old higher-precedence
behavior can be obtained by enabling the
HIGH_NOT_PRECEDENCE SQL
mode. (Added in MySQL 5.0.2)
mysql> SET sql_mode = '';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
-> 0
mysql> SET sql_mode = 'HIGH_NOT_PRECEDENCE';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
-> 1
IGNORE_SPACE
Allow spaces between a function name and the
“(” character. This causes
built-in function names to be treated as reserved words. As a
result, identifiers that are the same as function names must
be quoted as described in Section 8.2, “Schema Object Names”. For
example, because there is a
COUNT() function, the use of
count as a table name in the following
statement causes an error:
mysql> CREATE TABLE count (i INT);
ERROR 1064 (42000): You have an error in your SQL syntax
The table name should be quoted:
mysql> CREATE TABLE `count` (i INT);
Query OK, 0 rows affected (0.00 sec)
The IGNORE_SPACE SQL mode
applies to built-in functions, not to user-defined functions
or stored functions. It is always allowable to have spaces
after a UDF or stored function name, regardless of whether
IGNORE_SPACE is enabled.
For further discussion of
IGNORE_SPACE, see
Section 8.2.3, “Function Name Parsing and Resolution”.
NO_AUTO_CREATE_USER
Prevent the GRANT statement
from automatically creating new users if it would otherwise do
so, unless a nonempty password also is specified. (Added in
MySQL 5.0.2)
NO_AUTO_VALUE_ON_ZERO
NO_AUTO_VALUE_ON_ZERO
affects handling of AUTO_INCREMENT columns.
Normally, you generate the next sequence number for the column
by inserting either NULL or
0 into it.
NO_AUTO_VALUE_ON_ZERO
suppresses this behavior for 0 so that only
NULL generates the next sequence number.
This mode can be useful if 0 has been
stored in a table's AUTO_INCREMENT column.
(Storing 0 is not a recommended practice,
by the way.) For example, if you dump the table with
mysqldump and then reload it, MySQL
normally generates new sequence numbers when it encounters the
0 values, resulting in a table with
contents different from the one that was dumped. Enabling
NO_AUTO_VALUE_ON_ZERO before
reloading the dump file solves this problem.
mysqldump now automatically includes in its
output a statement that enables
NO_AUTO_VALUE_ON_ZERO, to
avoid this problem.
NO_BACKSLASH_ESCAPES
Disable the use of the backslash character
(“\”) as an escape character
within strings. With this mode enabled, backslash becomes an
ordinary character like any other. (Implemented in MySQL
5.0.1)
NO_DIR_IN_CREATE
When creating a table, ignore all INDEX
DIRECTORY and DATA DIRECTORY
directives. This option is useful on slave replication
servers.
NO_ENGINE_SUBSTITUTION
Control automatic substitution of the default storage engine
when a statement such as CREATE
TABLE or ALTER TABLE
specifies a storage engine that is disabled or not compiled
in. (Implemented in MySQL 5.0.8)
With NO_ENGINE_SUBSTITUTION
disabled, the default engine is used and a warning occurs if
the desired engine is known but disabled or not compiled in.
If the desired engine is invalid (not a known engine name), an
error occurs and the table is not created or altered.
With NO_ENGINE_SUBSTITUTION
enabled, an error occurs and the table is not created or
altered if the desired engine is unavailable for any reason
(whether disabled or invalid).
NO_FIELD_OPTIONS
Do not print MySQL-specific column options in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
NO_KEY_OPTIONS
Do not print MySQL-specific index options in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
NO_TABLE_OPTIONS
Do not print MySQL-specific table options (such as
ENGINE) in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
NO_UNSIGNED_SUBTRACTION
In integer subtraction operations, do not mark the result as
UNSIGNED if one of the operands is
unsigned. In other words, the result of a
subtraction is always signed whenever this mode is in effect,
even if one of the operands is unsigned. For
example, compare the type of column c2 in
table t1 with that of column
c2 in table t2:
mysql> SET SQL_MODE='';
mysql> CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);
mysql> CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| c2 | bigint(21) unsigned | | | 0 | |
+-------+---------------------+------+-----+---------+-------+
mysql> SET SQL_MODE='NO_UNSIGNED_SUBTRACTION';
mysql> CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t2;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| c2 | bigint(21) | | | 0 | |
+-------+------------+------+-----+---------+-------+
Note that this means that BIGINT UNSIGNED
is not 100% usable in all contexts. See
Section 11.9, “Cast Functions and Operators”.
mysql> SET SQL_MODE = '';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
| 18446744073709551615 |
+-------------------------+
mysql> SET SQL_MODE = 'NO_UNSIGNED_SUBTRACTION';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
| -1 |
+-------------------------+
NO_ZERO_DATE
In strict mode, don't allow '0000-00-00' as
a valid date. You can still insert zero dates with the
IGNORE option. When not in strict mode, the
date is accepted but a warning is generated. (Added in MySQL
5.0.2)
NO_ZERO_IN_DATE
In strict mode, do not accept dates where the year part is
nonzero but the month or day part is 0 (for example,
'0000-00-00' is legal but
'2010-00-01' and
'2010-01-00' are not). If used with the
IGNORE option, MySQL inserts a
'0000-00-00' date for any such date. When
not in strict mode, the date is accepted but a warning is
generated. (Added in MySQL 5.0.2)
ONLY_FULL_GROUP_BY
Do not allow queries for which the
SELECT list refers to
nonaggregated columns that are not named in the GROUP
BY clause. The following query is invalid with this
mode enabled because address is not named
in the GROUP BY clause:
SELECT name, address, MAX(age) FROM t GROUP BY name;
As of MySQL 5.0.23, this mode also restricts references to
nonaggregated columns in the HAVING clause
that are not named in the GROUP BY clause.
PIPES_AS_CONCAT
Treat || as a
string concatenation operator (same as
CONCAT()) rather than as a
synonym for OR.
REAL_AS_FLOAT
Treat REAL as a synonym for
FLOAT. By default, MySQL treats
REAL as a synonym for
DOUBLE.
STRICT_ALL_TABLES
Enable strict mode for all storage engines. Invalid data
values are rejected. Additional detail follows. (Added in
MySQL 5.0.2)
STRICT_TRANS_TABLES
Enable strict mode for transactional storage engines, and when
possible for nontransactional storage engines. Additional
details follow. (Implemented in MySQL 5.0.2)
Strict mode controls how MySQL handles input values that are
invalid or missing. A value can be invalid for several reasons.
For example, it might have the wrong data type for the column, or
it might be out of range. A value is missing when a new row to be
inserted does not contain a value for a
non-NULL column that has no explicit
DEFAULT clause in its definition. (For a
NULL column, NULL is
inserted if the value is missing.)
For transactional tables, an error occurs for invalid or missing
values in a statement when either of the
STRICT_ALL_TABLES or
STRICT_TRANS_TABLES modes are
enabled. The statement is aborted and rolled back.
For nontransactional tables, the behavior is the same for either
mode, if the bad value occurs in the first row to be inserted or
updated. The statement is aborted and the table remains unchanged.
If the statement inserts or modifies multiple rows and the bad
value occurs in the second or later row, the result depends on
which strict option is enabled:
For STRICT_ALL_TABLES, MySQL
returns an error and ignores the rest of the rows. However, in
this case, the earlier rows still have been inserted or
updated. This means that you might get a partial update, which
might not be what you want. To avoid this, it is best to use
single-row statements because these can be aborted without
changing the table.
For STRICT_TRANS_TABLES,
MySQL converts an invalid value to the closest valid value for
the column and insert the adjusted value. If a value is
missing, MySQL inserts the implicit default value for the
column data type. In either case, MySQL generates a warning
rather than an error and continues processing the statement.
Implicit defaults are described in
Section 10.1.4, “Data Type Default Values”.
Strict mode disallows invalid date values such as
'2004-04-31'. It does not disallow dates with
zero month or day parts such as '2004-04-00' or
“zero” dates. To disallow these as well, enable the
NO_ZERO_IN_DATE and
NO_ZERO_DATE SQL modes in
addition to strict mode.
If you are not using strict mode (that is, neither
STRICT_TRANS_TABLES nor
STRICT_ALL_TABLES is enabled),
MySQL inserts adjusted values for invalid or missing values and
produces warnings. In strict mode, you can produce this behavior
by using INSERT
IGNORE or UPDATE IGNORE. See
Section 12.5.5.37, “SHOW WARNINGS Syntax”.
Strict mode does not affect whether foreign key constraints are
checked. foreign_key_checks can
be used for that. (See
Section 5.1.4, “Session System Variables”.)
The following special modes are provided as shorthand for
combinations of mode values from the preceding list. All are
available in MySQL 5.0 beginning with version 5.0.0,
except for TRADITIONAL, which
was implemented in MySQL 5.0.2.
The descriptions include all mode values that are available in the
most recent version of MySQL. For older versions, a combination
mode does not include individual mode values that are not
available except in newer versions.
ANSI
Equivalent to REAL_AS_FLOAT,
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE. Before MySQL
5.0.3, ANSI also includes
ONLY_FULL_GROUP_BY.
As of MySQL 5.0.40, ANSI
mode also causes the server to return an error for queries
where a set function S with an
outer reference
S(outer_ref) SELECT * FROM t1 WHERE t1.a IN (SELECT MAX(t1.b) FROM t2 WHERE ...);
Here, MAX(t1.b) cannot
aggregated in the outer query because it appears in the
WHERE clause of that query. Standard SQL
requires an error in this situation. If
ANSI mode is not enabled,
the server treats
S(outer_ref)S(const)
See Section 1.8.3, “Running MySQL in ANSI Mode”.
DB2
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
MAXDB
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS,
NO_AUTO_CREATE_USER.
MSSQL
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
MYSQL323
Equivalent to
NO_FIELD_OPTIONS,
HIGH_NOT_PRECEDENCE.
MYSQL40
Equivalent to
NO_FIELD_OPTIONS,
HIGH_NOT_PRECEDENCE.
ORACLE
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS,
NO_AUTO_CREATE_USER.
POSTGRESQL
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
TRADITIONAL
Equivalent to
STRICT_TRANS_TABLES,
STRICT_ALL_TABLES,
NO_ZERO_IN_DATE,
NO_ZERO_DATE,
ERROR_FOR_DIVISION_BY_ZERO,
NO_AUTO_CREATE_USER.
MySQL Server supports a HELP
statement that returns online information from the MySQL Reference
manual (see Section 12.3.3, “HELP Syntax”). The proper operation of this
statement requires that the help tables in the
mysql database be initialized with help topic
information, which is done by processing the contents of the
fill_help_tables.sql script.
For a MySQL binary distribution on Unix, help table setup occurs
when you run mysql_install_db. For an RPM
distribution on Linux or binary distribution on Windows, help
table setup occurs as part of the MySQL installation process.
For a MySQL source distribution, you can find the
fill_help_tables.sql file in the
scripts directory. To load the file manually,
make sure that you have initialized the mysql
database by running mysql_install_db, and then
process the file with the mysql client as
follows:
shell> mysql -u root mysql < fill_help_tables.sql
If you are working with Bazaar and a MySQL development source
tree, the tree doesn't contain
fill_help_tables.sql. You can download the
proper file for your version of MySQL from
http://dev.mysql.com/doc/. After downloading and
uncompressing the file, process it with mysql
as just described.
5.1.9. Server Response to Signals
On Unix, signals can be sent to processes.
mysqld responds to signals sent to it as
follows:
SIGTERM causes the server to shut down.
SIGHUP causes the server to reload the
grant tables and flush the logs (like
FLUSH
PRIVILEGES and
FLUSH LOGS).
It also writes a status report to the error log that has this
format:
Status information:
Current dir: /var/mysql/data/
Running threads: 0 Stack size: 196608
Current locks:
Key caches:
default
Buffer_size: 8388600
Block_size: 1024
Division_limit: 100
Age_limit: 300
blocks used: 0
not flushed: 0
w_requests: 0
writes: 0
r_requests: 0
reads: 0
handler status:
read_key: 0
read_next: 0
read_rnd 0
read_first: 1
write: 0
delete 0
update: 0
Table status:
Opened tables: 5
Open tables: 0
Open files: 7
Open streams: 0
Alarm status:
Active alarms: 1
Max used alarms: 2
Next alarm time: 67
On some Mac OS X 10.3 versions, mysqld ignores
SIGHUP and SIGQUIT.
5.1.10. The Shutdown Process
The server shutdown process takes place as follows:
The shutdown process is initiated.
Server shutdown can be initiated several ways. For example, a
user with the SHUTDOWN
privilege can execute a mysqladmin shutdown
command. mysqladmin can be used on any
platform supported by MySQL. Other operating system-specific
shutdown initiation methods are possible as well: The server
shuts down on Unix when it receives a
SIGTERM signal. A server running as a
service on Windows shuts down when the services manager tells
it to.
The server creates a shutdown thread if necessary.
Depending on how shutdown was initiated, the server might
create a thread to handle the shutdown process. If shutdown
was requested by a client, a shutdown thread is created. If
shutdown is the result of receiving a
SIGTERM signal, the signal thread might
handle shutdown itself, or it might create a separate thread
to do so. If the server tries to create a shutdown thread and
cannot (for example, if memory is exhausted), it issues a
diagnostic message that appears in the error log:
Error: Can't create thread to kill server
The server stops accepting new connections.
To prevent new activity from being initiated during shutdown,
the server stops accepting new client connections. It does
this by closing the network connections to which it normally
listens for connections: the TCP/IP port, the Unix socket
file, the Windows named pipe, and shared memory on Windows.
The server terminates current activity.
For each thread that is associated with a client connection,
the connection to the client is broken and the thread is
marked as killed. Threads die when they notice that they are
so marked. Threads for idle connections die quickly. Threads
that currently are processing statements check their state
periodically and take longer to die. For additional
information about thread termination, see
Section 12.5.6.3, “KILL Syntax”, in particular for the instructions
about killed REPAIR TABLE or
OPTIMIZE TABLE operations on
MyISAM tables.
For threads that have an open transaction, the transaction is
rolled back. Note that if a thread is updating a
nontransactional table, an operation such as a multiple-row
UPDATE or
INSERT may leave the table
partially updated, because the operation can terminate before
completion.
If the server is a master replication server, threads
associated with currently connected slaves are treated like
other client threads. That is, each one is marked as killed
and exits when it next checks its state.
If the server is a slave replication server, the I/O and SQL
threads, if active, are stopped before client threads are
marked as killed. The SQL thread is allowed to finish its
current statement (to avoid causing replication problems), and
then stops. If the SQL thread was in the middle of a
transaction at this point, the transaction is rolled back.
Storage engines are shut down or closed.
At this stage, the table cache is flushed and all open tables
are closed.
Each storage engine performs any actions necessary for tables
that it manages. For example, MyISAM
flushes any pending index writes for a table.
InnoDB flushes its buffer pool to disk
(starting from 5.0.5: unless
innodb_fast_shutdown is 2),
writes the current LSN to the tablespace, and terminates its
own internal threads.
The server exits.
MySQL has several different logs that can help you find out what is
going on inside mysqld.
By default, all log files are created in the
mysqld data directory. You can force
mysqld to close and reopen the log files (or in
some cases switch to a new log) by flushing the logs. Log flushing
occurs when you issue a FLUSH
LOGS statement or execute a mysqladmin
flush-logs, mysqladmin refresh,
mysqldump --flush-logs, or mysqldump
--master-data command. See Section 12.5.6.2, “FLUSH Syntax”,
Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In
addition, the binary log is flushed when its size reaches the value
of the max_binlog_size system
variable.
If you are using MySQL replication capabilities, slave replication
servers maintain additional log files called relay logs.
Chapter 16, Replication, discusses relay log contents and
configuration.
See Section 5.5.6.1, “Administrator Guidelines for Password Security”, for information about
keeping logs secure.
MySQL Enterprise
The MySQL Enterprise Monitor provides a number of advisors
specifically related to the various log files. For more
information, see
http://www.mysql.com/products/enterprise/advisors.html.
The error log contains information indicating when
mysqld was started and stopped and also any
critical errors that occur while the server is running. If
mysqld notices a table that needs to be
automatically checked or repaired, it writes a message to the
error log.
On some operating systems, the error log contains a stack trace if
mysqld dies. The trace can be used to determine
where mysqld died. See
MySQL
Internals: Porting.
You can specify where mysqld writes the error
log with the
--log-error[=file_name]
option. If no file_name value is given,
mysqld uses the name
host_name.errFLUSH
LOGS, the error log is renamed with the suffix
-old and mysqld creates a
new empty log file. (No renaming occurs if the
--log-error option was not given to
mysqld.)
If you do not specify --log-error,
or (on Windows) if you use the
--console option, errors are
written to stderr, the standard error output.
Usually this is your terminal.
On Windows, error output is always written to the
.err file if
--console is not given.
In addition, on Windows, events and error messages are written to
the Windows Event Log within the Application log. Entries marked
as Warning and Note are
written to the Event Log, but informational messages (such as
information statements from individual storage engines) are not
copied to the Event Log. The log entries will have a source of
MySQL. You cannot disable writing information
to the Windows Event Log.
The --log-warnings option or
log_warnings system variable can
be used to control warning logging to the error log. The default
value is enabled (1). Warning logging can be disabled using a
value of 0. If the value is greater than 1, aborted connections
are written to the error log. See
Section B.5.2.11, “Communication Errors and Aborted Connections”.
If you use mysqld_safe to start
mysqld, mysqld_safe arranges
for mysqld to write error messages to a log
file. If you specify a file name via
--log-error to
mysqld_safe or mysqld, that
file name is used. Otherwise, mysqld_safe uses
the default error log file.
If mysqld_safe is used to start
mysqld and mysqld dies
unexpectedly, mysqld_safe notices that it needs
to restart mysqld and writes a
restarted mysqld message to the error log.
5.2.2. The General Query Log
The general query log is a general record of what
mysqld is doing. The server writes information
to this log when clients connect or disconnect, and it logs each
SQL statement received from clients. The general query log can be
very useful when you suspect an error in a client and want to know
exactly what the client sent to mysqld.
mysqld writes statements to the query log in
the order that it receives them, which might differ from the order
in which they are executed. This logging order contrasts to the
binary log, for which statements are written after they are
executed but before any locks are released. (Also, the query log
contains all statements, whereas the binary log does not contain
statements that only select data.)
To enable the general query log, start mysqld
with the
--log[=file_name]
or -l [file_name]
option.
If no file_name value is given for
--log or -l, the
default name is
host_name.log
Server restarts and log flushing do not cause a new general query
log file to be generated (although flushing closes and reopens
it). On Unix, you can rename the file and create a new one by
using the following commands:
shell> mv host_name.log host_name-old.log
shell> mysqladmin flush-logs
shell> cp host_name-old.log backup-directory
shell> rm host_name-old.log
Before 5.0.17, you cannot rename a log file on Windows while the
server has it open. You must stop the server and rename the file,
and then restart the server to create a new log file. As of
5.0.17, this applies only to the error log. However, a stop and
restart can be avoided by using
FLUSH LOGS, which
causes the server to rename the error log with an
-old suffix and open a new error log.
The general query log should be protected because logged
statements might contain passwords. See
Section 5.5.6.1, “Administrator Guidelines for Password Security”.
The binary log contains all statements that update data or
potentially could have updated it (for example, a
DELETE which matched no rows).
Statements are stored in the form of “events” that
describe the modifications. The binary log also contains
information about how long each statement took that updated data.
The binary log has two important purposes:
For replication, the binary log is used on master replication
servers as a record of the statements to be sent to slave
servers. The master server sends the events contained in its
binary log to its slaves, which execute those events to make
the same data changes that were made on the master. See
Section 16.4, “Replication Implementation”.
Certain data recovery operations require use of the binary
log. After a backup file has been restored, the events in the
binary log that were recorded after the backup was made are
re-executed. These events bring databases up to date from the
point of the backup. See
Section 6.4, “Point-in-Time (Incremental) Recovery Using the Binary Log”.
Note
The binary log has replaced the old update log, which is no
longer available as of MySQL 5.0. The binary log contains all
information that is available in the update log in a more
efficient format and in a manner that is transaction-safe. If
you are using transactions, you must use the MySQL binary log
for backups instead of the old update log.
Running the server with the binary log enabled makes performance
about 1% slower. However, the benefits of the binary log in
allowing you to set up replication and for restore operations
generally outweigh this minor performance decrement.
For information about server options and variables affecting the
operation of binary logging, see
Section 16.1.2.4, “Binary Log Options and Variables”.
The binary log is not used for statements such as
SELECT or
SHOW that do not modify data. If
you want to log all statements (for example, to identify a problem
query), use the general query log. See
Section 5.2.2, “The General Query Log”.
The binary log should be protected because logged statements might
contain passwords. See Section 5.5.6.1, “Administrator Guidelines for Password Security”.
MySQL Enterprise
The binary log can also be used to track significant DDL events.
Analyzing the binary log in this way is an integral part of the
MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
To enable the binary log, start the server with the
--log-bin[=base_name]
option. mysqld writes a log file containing all
SQL statements that update data (both DDL and DML statements). If
no base_name value is given, the
default name is the value of the pid-file
option (which by default is the name of host machine) followed by
-bin. If the basename is given, the server
writes the file in the data directory unless the basename is given
with a leading absolute path name to specify a different
directory. It is recommended that you specify a basename; see
Additional Known Issues, for the reason.
Note
From MySQL 5.0.41 through 5.0.52, “mysql” was used
when no base_name was specified. Also
in these versions, a path given as part of the
--log-bin options was treated as
absolute rather than relative. The previous behaviors were
restored in MySQL 5.0.54. (See Bug#28603 and Bug#28597.)
If you supply an extension in the log name (for example,
--log-bin=base_name.extension),
the extension is silently removed and ignored.
mysqld appends a numeric extension to the
binary log basename to generate binary log file names. The number
increases each time the server creates a new log file, thus
creating an ordered series of files. The server creates a new file
in the series each time it starts or flushes the logs. The server
also creates a new binary log file automatically after the current
log's size reaches
max_binlog_size. A binary log
file may become larger than
max_binlog_size if you are using
large transactions because a transaction is written to the file in
one piece, never split between files.
To keep track of which binary log files have been used,
mysqld also creates a binary log index file
that contains the names of all used binary log files. By default,
this has the same basename as the binary log file, with the
extension '.index'. You can change the name of
the binary log index file with the
--log-bin-index[=file_name]
option. You should not manually edit this file while
mysqld is running; doing so would confuse
mysqld.
The server evaluates the
--binlog-do-db and
--binlog-ignore-db options in the
same way as it does the
--replicate-do-db and
--replicate-ignore-db options. For
information about how this is done, see
Section 16.4.3.1, “Evaluation of Database-Level Replication and Binary Logging Options”.
A replication slave server by default does not write to its own
binary log any data modifications that are received from the
replication master. To log these modifications, start the slave
with the --log-slave-updates option
(see Section 16.1.2.3, “Replication Slave Options and Variables”).
You can delete all binary log files with the
RESET MASTER statement, or a subset
of them with PURGE BINARY LOGS. See
Section 12.5.6.5, “RESET Syntax”, and Section 12.6.1.1, “PURGE BINARY LOGS Syntax”.
If you are using replication, you should not delete old binary log
files on the master until you are sure that no slave still needs
to use them. For example, if your slaves never run more than three
days behind, once a day you can execute mysqladmin
flush-logs on the master and then remove any logs that
are more than three days old. You can remove the files manually,
but it is preferable to use PURGE BINARY
LOGS, which also safely updates the binary log index
file for you (and which can take a date argument). See
Section 12.6.1.1, “PURGE BINARY LOGS Syntax”.
A client that has the SUPER
privilege can disable binary logging of its own statements by
using a SET sql_log_bin=0 statement. See
Section 5.1.4, “Session System Variables”.
You can display the contents of binary log files with the
mysqlbinlog utility. This can be useful when
you want to reprocess statements in the log for a recovery
operation. For example, you can update a MySQL server from the
binary log as follows:
shell> mysqlbinlog log_file | mysql -h server_name
mysqlbinlog also can be used to display relay
log file contents because they are written using the same format
as binary log files. For more information on the
mysqlbinlog utility and how to use it, see
Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”. For more information about the
binary log and recovery operations, see
Section 6.4, “Point-in-Time (Incremental) Recovery Using the Binary Log”.
Binary logging is done immediately after a statement completes but
before any locks are released or any commit is done. This ensures
that the log is logged in execution order.
Updates to nontransactional tables are stored in the binary log
immediately after execution. In MySQL 5.0.53 and earlier versions
of MySQL 5.0, an
UPDATE statement using a stored
function that modified a nontransactional table was not logged if
it failed, and an
INSERT ... ON
DUPLICATE KEY UPDATE statement that encountered a
duplicate key constraint — but did not actually change any
data — was not logged. Beginning with MySQL 5.0.54, both of
these statements are written to the binary log. (Bug#23333)
Within an uncommitted transaction, all updates
(UPDATE,
DELETE, or
INSERT) that change transactional
tables such as BDB or InnoDB
tables are cached until a COMMIT
statement is received by the server. At that point,
mysqld writes the entire transaction to the
binary log before the COMMIT is
executed. When the thread that handles the transaction starts, it
allocates a buffer of
binlog_cache_size to buffer
statements. If a statement is bigger than this, the thread opens a
temporary file to store the transaction. The temporary file is
deleted when the thread ends.
The Binlog_cache_use status
variable shows the number of transactions that used this buffer
(and possibly a temporary file) for storing statements. The
Binlog_cache_disk_use status
variable shows how many of those transactions actually had to use
a temporary file. These two variables can be used for tuning
binlog_cache_size to a large
enough value that avoids the use of temporary files.
The max_binlog_cache_size system
variable (default 4GB, which is also the maximum) can be used to
restrict the total size used to cache a multiple-statement
transaction. If a transaction is larger than this many bytes, it
fails and rolls back. The minimum value is 4096.
Modifications to nontransactional tables cannot be rolled back. If
a transaction that is rolled back includes modifications to
nontransactional tables, the entire transaction is logged with a
ROLLBACK
statement at the end to ensure that the modifications to those
tables are replicated.
If you are using the binary log and row based logging, concurrent
inserts are converted to normal inserts for CREATE ...
SELECT or
INSERT ...
SELECT statement. This is done to ensure that you can
re-create an exact copy of your tables by applying the log during
a backup operation. If you are using statement-based logging, the
original statement is written to the log.
The binary log format has some known limitations that can affect
recovery from backups. See Section 16.3.1, “Replication Features and Issues”.
Binary logging for stored programs is done as described in
Section 18.5, “Binary Logging of Stored Programs”.
Note that the binary log format differs in MySQL 5.0
from previous versions of MySQL, due to enhancements in
replication. See Section 16.3.2, “Replication Compatibility Between MySQL Versions”.
Writes to the binary log file and binary log index file are
handled in the same way as writes to MyISAM
tables. See Section B.5.4.3, “How MySQL Handles a Full Disk”.
By default, the binary log is not synchronized to disk at each
write. So if the operating system or machine (not only the MySQL
server) crashes, there is a chance that the last statements of the
binary log are lost. To prevent this, you can make the binary log
be synchronized to disk after every N
writes to the binary log, with the
sync_binlog system variable. See
Section 5.1.3, “Server System Variables”. 1 is the safest value
for sync_binlog, but also the
slowest. Even with sync_binlog
set to 1, there is still the chance of an inconsistency between
the table content and binary log content in case of a crash. For
example, if you are using InnoDB tables and the
MySQL server processes a COMMIT
statement, it writes the whole transaction to the binary log and
then commits this transaction into InnoDB. If
the server crashes between those two operations, the transaction
is rolled back by InnoDB at restart but still
exists in the binary log. This problem can be solved with the
--innodb_safe_binlog option, which
adds consistency between the content of InnoDB
tables and the binary log. (Note:
--innodb_safe_binlog is unneeded as
of MySQL 5.0; it was made obsolete by the introduction of XA
transaction support.)
For this option to provide a greater degree of safety, the MySQL
server should also be configured to synchronize the binary log and
the InnoDB logs to disk at every transaction.
The InnoDB logs are synchronized by default,
and sync_binlog=1 can be used to synchronize
the binary log. The effect of this option is that at restart after
a crash, after doing a rollback of transactions, the MySQL server
cuts rolled back InnoDB transactions from the
binary log. This ensures that the binary log reflects the exact
data of InnoDB tables, and so, that the slave
remains in synchrony with the master (not receiving a statement
which has been rolled back).
Note that --innodb_safe_binlog can
be used even if the MySQL server updates other storage engines
than InnoDB. Only statements and transactions
that affect InnoDB tables are subject to
removal from the binary log at InnoDB's crash
recovery. If the MySQL server discovers at crash recovery that the
binary log is shorter than it should have been, it lacks at least
one successfully committed InnoDB transaction.
This should not happen if sync_binlog=1 and the
disk/file system do an actual sync when they are requested to
(some don't), so the server prints an error message The
binary log <name> is shorter than its expected
size. In this case, this binary log is not correct and
replication should be restarted from a fresh snapshot of the
master's data.
For MySQL 5.0.46, the session values of the following system
variables are written to the binary log and honored by the
replication slave when parsing the binary log:
5.2.4. The Slow Query Log
The slow query log consists of all SQL statements that took more
than long_query_time seconds to
execute. The time to acquire the initial table locks is not
counted as execution time. mysqld writes a
statement to the slow query log after it has been executed and
after all locks have been released, so log order might be
different from execution order. The minimum and default values of
long_query_time are 1 and 10,
respectively.
To enable the slow query log, start mysqld with
the
--log-slow-queries[=file_name]
option.
If no file_name value is given for
--log-slow-queries, the default
name is
host_name-slow.log
The slow query log can be used to find queries that take a long
time to execute and are therefore candidates for optimization.
However, examining a long slow query log can become a difficult
task. To make this easier, you can process a slow query log file
using the mysqldumpslow command to summarize
the queries that appear in the log. See
Section 4.6.8, “mysqldumpslow — Summarize Slow Query Log Files”.
In MySQL 5.0, queries that do not use indexes are
logged in the slow query log if the
--log-queries-not-using-indexes
option is specified. See Section 5.1.2, “Server Command Options”.
MySQL Enterprise
Excessive table scans are indicative of missing or poorly
optimized indexes. Using an advisor specifically designed for
the task, the MySQL Enterprise Monitor can identify such
problems and offer advice on resolution. For more information,
see http://www.mysql.com/products/enterprise/advisors.html.
In MySQL 5.0, the
--log-slow-admin-statements server
option enables you to request logging of slow administrative
statements such as OPTIMIZE TABLE,
ANALYZE TABLE, and
ALTER TABLE to the slow query log.
Queries handled by the query cache are not added to the slow query
log, nor are queries that would not benefit from the presence of
an index because the table has zero rows or one row.
Replication slaves do not write replicated queries to the slow
query log, even if the same queries were written to the slow query
log on the master. This is a known issue which we intend to fix in
a future version of MySQL. (Bug#23300)
The slow query log should be protected because logged statements
might contain passwords. See
Section 5.5.6.1, “Administrator Guidelines for Password Security”.
5.2.5. Server Log Maintenance
MySQL Server can create a number of different log files that make
it easy to see what is going on. See
Section 5.2, “MySQL Server Logs”. However, you must clean up these
files regularly to ensure that the logs do not take up too much
disk space.
When using MySQL with logging enabled, you may want to back up and
remove old log files from time to time and tell MySQL to start
logging to new files. See Section 6.2, “Database Backup Methods”.
On a Linux (Red Hat) installation, you can use the
mysql-log-rotate script for this. If you
installed MySQL from an RPM distribution, this script should have
been installed automatically. You should be careful with this
script if you are using the binary log for replication. You should
not remove binary logs until you are certain that their contents
have been processed by all slaves.
On other systems, you must install a short script yourself that
you start from cron (or its equivalent) for
handling log files.
For the binary log, you can set the
expire_logs_days system variable
to expire binary log files automatically after a given number of
days (see Section 5.1.3, “Server System Variables”). If you are
using replication, you should set the variable no lower than the
maximum number of days your slaves might lag behind the master.
You can force MySQL to start using new log files by flushing the
logs. Log flushing occurs when you issue a
FLUSH LOGS
statement or execute a mysqladmin flush-logs,
mysqladmin refresh, mysqldump
--flush-logs, or mysqldump
--master-data command. See Section 12.5.6.2, “FLUSH Syntax”,
Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In
addition, the binary log is flushed when its size reaches the
value of the max_binlog_size
system variable.
A log flushing operation does the following:
If general query logging
(--log) or slow query logging
(--log-slow-queries) to a log
file is enabled, the server closes and reopens the general
query log file or slow query log file.
If binary logging (--log-bin)
is used, the server closes the current log file and opens a
new log file with the next sequence number.
If the server was given an error log file name with the
--log-error option, it renames
the error log with the suffix -old and
creates a new empty error log file.
The server creates a new binary log file when you flush the logs.
However, it just closes and reopens the general and slow query log
files. To cause new files to be created on Unix, rename the
current logs before flushing them. At flush time, the server will
open new logs with the original names. For example, if the general
and slow query logs are named mysql.log and
mysql-slow.log, you can use a series of
commands like this:
shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mv mysql-slow.log mysql-slow.old
shell> mysqladmin flush-logs
At this point, you can make a backup of
mysql.old and
mysql-slow.log and then remove them from
disk.
Before 5.0.17, you cannot rename a log file on Windows while the
server has it open. You must stop the server and rename the file,
and then restart the server to create a new log file. As of
5.0.17, this applies only to the error log. However, a stop and
restart can be avoided by using
FLUSH LOGS, which
causes the server to rename the error log with an
-old suffix and open a new error log.
The session sql_log_off variable
can be set to ON or OFF to
disable or enable general query logging for the current
connection.
5.3. General Security Issues
This section describes some general security issues to be aware of
and what you can do to make your MySQL installation more secure
against attack or misuse. For information specifically about the
access control system that MySQL uses for setting up user accounts
and checking database access, see
Section 5.4, “The MySQL Access Privilege System”.
For answers to some questions that are often asked about MySQL
Server security issues, see Section A.9, “MySQL 5.0 FAQ — Security”.
5.3.1. General Security Guidelines
Anyone using MySQL on a computer connected to the Internet should
read this section to avoid the most common security mistakes.
In discussing security, we emphasize the necessity of fully
protecting the entire server host (not just the MySQL server)
against all types of applicable attacks: eavesdropping, altering,
playback, and denial of service. We do not cover all aspects of
availability and fault tolerance here.
MySQL uses security based on Access Control Lists (ACLs) for all
connections, queries, and other operations that users can attempt
to perform. There is also support for SSL-encrypted connections
between MySQL clients and servers. Many of the concepts discussed
here are not specific to MySQL at all; the same general ideas
apply to almost all applications.
When running MySQL, follow these guidelines whenever possible:
Do not ever give anyone (except MySQL
root accounts) access to the
user table in the mysql
database! This is critical.
Learn the MySQL access privilege system. The
GRANT and
REVOKE statements are used for
controlling access to MySQL. Do not grant more privileges than
necessary. Never grant privileges to all hosts.
Checklist:
Try mysql -u root. If you are able to
connect successfully to the server without being asked for
a password, anyone can connect to your MySQL server as the
MySQL root user with full privileges!
Review the MySQL installation instructions, paying
particular attention to the information about setting a
root password. See
Section 2.17.3, “Securing the Initial MySQL Accounts”.
Use the SHOW GRANTS
statement to check which accounts have access to what.
Then use the REVOKE
statement to remove those privileges that are not
necessary.
Do not store any plain-text passwords in your database. If
your computer becomes compromised, the intruder can take the
full list of passwords and use them. Instead, use
MD5(),
SHA1(), or some other one-way
hashing function and store the hash value.
Do not choose passwords from dictionaries. Special programs
exist to break passwords. Even passwords like
“xfish98” are very bad. Much better is
“duag98” which contains the same word
“fish” but typed one key to the left on a
standard QWERTY keyboard. Another method is to use a password
that is taken from the first characters of each word in a
sentence (for example, “Mary had a little lamb”
results in a password of “Mhall”). The password
is easy to remember and type, but difficult to guess for
someone who does not know the sentence.
MySQL Enterprise
MySQL Enterprise subscribers can find an example of a
function that checks password security in the Knowledge Base
article,
Checking Password Complexity. To subscribe to MySQL
Enterprise see
http://www.mysql.com/products/enterprise/advisors.html.
Invest in a firewall. This protects you from at least 50% of
all types of exploits in any software. Put MySQL behind the
firewall or in a demilitarized zone (DMZ).
Checklist:
Try to scan your ports from the Internet using a tool such
as nmap. MySQL uses port 3306 by
default. This port should not be accessible from untrusted
hosts. Another simple way to check whether or not your
MySQL port is open is to try the following command from
some remote machine, where
server_host is the host name or
IP number of the host on which your MySQL server runs:
shell> telnet server_host 3306
If you get a connection and some garbage characters, the
port is open, and should be closed on your firewall or
router, unless you really have a good reason to keep it
open. If telnet hangs or the connection
is refused, the port is blocked, which is how you want it
to be.
Do not trust any data entered by users of your applications.
They can try to trick your code by entering special or escaped
character sequences in Web forms, URLs, or whatever
application you have built. Be sure that your application
remains secure if a user enters something like
“; DROP DATABASE mysql;”. This
is an extreme example, but large security leaks and data loss
might occur as a result of hackers using similar techniques,
if you do not prepare for them.
A common mistake is to protect only string data values.
Remember to check numeric data as well. If an application
generates a query such as SELECT * FROM table WHERE
ID=234 when a user enters the value
234, the user can enter the value
234 OR 1=1 to cause the application to
generate the query SELECT * FROM table WHERE ID=234
OR 1=1. As a result, the server retrieves every row
in the table. This exposes every row and causes excessive
server load. The simplest way to protect from this type of
attack is to use single quotes around the numeric constants:
SELECT * FROM table WHERE ID='234'. If the
user enters extra information, it all becomes part of the
string. In a numeric context, MySQL automatically converts
this string to a number and strips any trailing nonnumeric
characters from it.
Sometimes people think that if a database contains only
publicly available data, it need not be protected. This is
incorrect. Even if it is allowable to display any row in the
database, you should still protect against denial of service
attacks (for example, those that are based on the technique in
the preceding paragraph that causes the server to waste
resources). Otherwise, your server becomes unresponsive to
legitimate users.
Checklist:
Try to enter single and double quote marks
(“'” and
“"”) in all of your Web
forms. If you get any kind of MySQL error, investigate the
problem right away.
Try to modify dynamic URLs by adding
%22
(“"”),
%23
(“#”), and
%27
(“'”) to them.
Try to modify data types in dynamic URLs from numeric to
character types using the characters shown in the previous
examples. Your application should be safe against these
and similar attacks.
Try to enter characters, spaces, and special symbols
rather than numbers in numeric fields. Your application
should remove them before passing them to MySQL or else
generate an error. Passing unchecked values to MySQL is
very dangerous!
Check the size of data before passing it to MySQL.
Have your application connect to the database using a user
name different from the one you use for administrative
purposes. Do not give your applications any access
privileges they do not need.
Many application programming interfaces provide a means of
escaping special characters in data values. Properly used,
this prevents application users from entering values that
cause the application to generate statements that have a
different effect than you intend:
MySQL C API: Use the
mysql_real_escape_string()
API call.
MySQL++: Use the escape and
quote modifiers for query streams.
PHP: Use the
mysql_real_escape_string() function
(available as of PHP 4.3.0, prior to that PHP version use
mysql_escape_string(), and prior to
PHP 4.0.3, use addslashes() ). Note
that only mysql_real_escape_string()
is character set-aware; the other functions can be
“bypassed” when using (invalid) multi-byte
character sets. In PHP 5, you can use the
mysqli extension, which supports the
improved MySQL authentication protocol and passwords, as
well as prepared statements with placeholders.
Perl DBI: Use placeholders or the
quote() method.
Ruby DBI: Use placeholders or the
quote() method.
Java JDBC: Use a PreparedStatement
object and placeholders.
Other programming interfaces might have similar capabilities.
Do not transmit plain (unencrypted) data over the Internet.
This information is accessible to everyone who has the time
and ability to intercept it and use it for their own purposes.
Instead, use an encrypted protocol such as SSL or SSH. MySQL
supports internal SSL connections as of version 4.0. Another
technique is to use SSH port-forwarding to create an encrypted
(and compressed) tunnel for the communication.
Learn to use the tcpdump and
strings utilities. In most cases, you can
check whether MySQL data streams are unencrypted by issuing a
command like the following:
shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings
This works under Linux and should work with small
modifications under other systems.
Warning
If you do not see plaintext data, this does not always mean
that the information actually is encrypted. If you need high
security, you should consult with a security expert.
5.3.2. Making MySQL Secure Against Attackers
When you connect to a MySQL server, you should use a password. The
password is not transmitted in clear text over the connection.
Password handling during the client connection sequence was
upgraded in MySQL 4.1.1 to be very secure. If you are still using
pre-4.1.1-style passwords, the encryption algorithm is not as
strong as the newer algorithm. With some effort, a clever attacker
who can sniff the traffic between the client and the server can
crack the password. (See Section 5.5.6.3, “Password Hashing in MySQL”, for a
discussion of the different password handling methods.)
MySQL Enterprise
The MySQL Enterprise Monitor enforces best practices for
maximizing the security of your servers. For more information,
see http://www.mysql.com/products/enterprise/advisors.html.
All other information is transferred as text, and can be read by
anyone who is able to watch the connection. If the connection
between the client and the server goes through an untrusted
network, and you are concerned about this, you can use the
compressed protocol to make traffic much more difficult to
decipher. You can also use MySQL's internal SSL support to make
the connection even more secure. See
Section 5.5.7, “Using SSL for Secure Connections”. Alternatively, use SSH to
get an encrypted TCP/IP connection between a MySQL server and a
MySQL client. You can find an Open Source SSH client at
http://www.openssh.org/, and a commercial SSH
client at http://www.ssh.com/.
To make a MySQL system secure, you should strongly consider the
following suggestions:
Require all MySQL accounts to have a password. A client
program does not necessarily know the identity of the person
running it. It is common for client/server applications that
the user can specify any user name to the client program. For
example, anyone can use the mysql program
to connect as any other person simply by invoking it as
mysql -u other_user
db_name if
other_user has no password. If all
accounts have a password, connecting using another user's
account becomes much more difficult.
For a discussion of methods for setting passwords, see
Section 5.5.5, “Assigning Account Passwords”.
Never run the MySQL server as the Unix root
user. This is extremely dangerous, because any user with the
FILE privilege is able to cause
the server to create files as root (for
example, ~root/.bashrc). To prevent this,
mysqld refuses to run as
root unless that is specified explicitly
using the --user=root option.
mysqld can (and should) be run as an
ordinary, unprivileged user instead. You can create a separate
Unix account named mysql to make everything
even more secure. Use this account only for administering
MySQL. To start mysqld as a different Unix
user, add a user option that specifies the
user name in the [mysqld] group of the
my.cnf option file where you specify
server options. For example:
[mysqld]
user=mysql
This causes the server to start as the designated user whether
you start it manually or by using
mysqld_safe or
mysql.server. For more details, see
Section 5.3.5, “How to Run MySQL as a Normal User”.
Running mysqld as a Unix user other than
root does not mean that you need to change
the root user name in the
user table. User names for MySQL
accounts have nothing to do with user names for Unix
accounts.
Do not allow the use of symlinks to tables. (This capability
can be disabled with the
--skip-symbolic-links
option.) This is especially important if you run
mysqld as root, because
anyone that has write access to the server's data directory
then could delete any file in the system! See
Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”.
Make sure that the only Unix user account with read or write
privileges in the database directories is the account that is
used for running mysqld.
Do not grant the PROCESS or
SUPER privilege to
nonadministrative users. The output of mysqladmin
processlist and SHOW
PROCESSLIST shows the text of any statements
currently being executed, so any user who is allowed to see
the server process list might be able to see statements issued
by other users such as UPDATE user SET
password=PASSWORD('not_secure').
mysqld reserves an extra connection for
users who have the SUPER
privilege, so that a MySQL root user can
log in and check server activity even if all normal
connections are in use.
The SUPER privilege can be used
to terminate client connections, change server operation by
changing the value of system variables, and control
replication servers.
Do not grant the FILE privilege
to nonadministrative users. Any user that has this privilege
can write a file anywhere in the file system with the
privileges of the mysqld daemon. To make
this a bit safer, files generated with
SELECT ... INTO
OUTFILE do not overwrite existing files and are
writable by everyone.
The FILE privilege may also be
used to read any file that is world-readable or accessible to
the Unix user that the server runs as. With this privilege,
you can read any file into a database table. This could be
abused, for example, by using LOAD
DATA to load /etc/passwd into a
table, which then can be displayed with
SELECT.
If you do not trust your DNS, you should use IP numbers rather
than host names in the grant tables. In any case, you should
be very careful about creating grant table entries using host
name values that contain wildcards.
If you want to restrict the number of connections allowed to a
single account, you can do so by setting the
max_user_connections variable
in mysqld. The
GRANT statement also supports
resource control options for limiting the extent of server use
allowed to an account. See Section 12.5.1.3, “GRANT Syntax”.
--ssl*
Options that begin with --ssl
specify whether to allow clients to connect via SSL and
indicate where to find SSL keys and certificates. See
Section 5.5.7.3, “SSL Command Options”.
5.3.3. Security-Related mysqld Options
The following mysqld options affect security:
Table 5.6. Security Option/Variable Summary
--allow-suspicious-udfs
This option controls whether user-defined functions that have
only an xxx symbol for the main function
can be loaded. By default, the option is off and only UDFs
that have at least one auxiliary symbol can be loaded; this
prevents attempts at loading functions from shared object
files other than those containing legitimate UDFs. For MySQL
5.0, this option was added in MySQL 5.0.3. See
Section 21.2.2.6, “User-Defined Function Security Precautions”.
--local-infile[={0|1}]
If you start the server with
--local-infile=0, clients
cannot use LOCAL in
LOAD DATA statements. See
Section 5.3.4, “Security Issues with LOAD
DATA LOCAL”.
--old-passwords
Force the server to generate short (pre-4.1) password hashes
for new passwords. This is useful for compatibility when the
server must support older client programs. See
Section 5.5.6.3, “Password Hashing in MySQL”.
MySQL Enterprise
The MySQL Enterprise Monitor offers advice on the security
implications of using this option. For subscription
information, see
http://www.mysql.com/products/enterprise/advisors.html.
--safe-show-database
(OBSOLETE)
In previous versions of MySQL, this option caused the
SHOW DATABASES statement to
display the names of only those databases for which the user
had some kind of privilege. In MySQL 5.0, this
option is no longer available as this is now the default
behavior, and there is a SHOW
DATABASES privilege that can be used to control
access to database names on a per-account basis. See
Section 12.5.1.3, “GRANT Syntax”.
--safe-user-create
If this option is enabled, a user cannot create new MySQL
users by using the GRANT
statement unless the user has the
INSERT privilege for the
mysql.user table or any column in the
table. If you want a user to have the ability to create new
users that have those privileges that the user has the right
to grant, you should grant the user the following privilege:
GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This ensures that the user cannot change any privilege columns
directly, but has to use the
GRANT statement to give
privileges to other users.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1)
passwords.
The mysql client also has a
--secure-auth option, which
prevents connections to a server if the server requires a
password in old format for the client account.
--secure-file-priv=path
This option limits the effect of the
LOAD_FILE() function and the
LOAD DATA and
SELECT ... INTO
OUTFILE statements to work only with files in the
specified directory.
This option was added in MySQL 5.0.38.
--skip-grant-tables
This option causes the server not to use the privilege system
at all. This gives anyone with access to the server
unrestricted access to all
databases. You can cause a running server to start
using the grant tables again by executing mysqladmin
flush-privileges or mysqladmin
reload command from a system shell, or by issuing a
MySQL FLUSH
PRIVILEGES statement. This option also suppresses
loading of user-defined functions (UDFs).
--skip-name-resolve
Host names are not resolved. All Host
column values in the grant tables must be IP numbers or
localhost.
--skip-networking
Do not allow TCP/IP connections over the network. All
connections to mysqld must be made via Unix
socket files.
--skip-show-database
With this option, the SHOW
DATABASES statement is allowed only to users who
have the SHOW DATABASES
privilege, and the statement displays all database names.
Without this option, SHOW
DATABASES is allowed to all users, but displays each
database name only if the user has the
SHOW DATABASES privilege or
some privilege for the database. Note that any global
privilege is a privilege for the database.
5.3.4. Security Issues with LOAD
DATA LOCAL
The LOAD DATA statement can load a
file that is located on the server host, or it can load a file
that is located on the client host when the
LOCAL keyword is specified.
There are two potential security issues with supporting the
LOCAL version of LOAD
DATA statements:
The transfer of the file from the client host to the server
host is initiated by the MySQL server. In theory, a patched
server could be built that would tell the client program to
transfer a file of the server's choosing rather than the file
named by the client in the LOAD
DATA statement. Such a server could access any file
on the client host to which the client user has read access.
In a Web environment where the clients are connecting from a
Web server, a user could use
LOAD DATA
LOCAL to read any files that the Web server process
has read access to (assuming that a user could run any command
against the SQL server). In this environment, the client with
respect to the MySQL server actually is the Web server, not
the remote program being run by the user who connects to the
Web server.
To deal with these problems, we changed how
LOAD DATA
LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2
(4.0.13 on Windows):
By default, all MySQL clients and libraries in binary
distributions are compiled with the
--enable-local-infile option, to be
compatible with MySQL 3.23.48 and before.
If you build MySQL from source but do not invoke
configure with the
--enable-local-infile option,
LOAD DATA
LOCAL cannot be used by any client unless it is
written explicitly to invoke
mysql_options(...
MYSQL_OPT_LOCAL_INFILE, 0). See
Section 20.8.3.49, “mysql_options()”.
You can disable all
LOAD DATA
LOCAL commands from the server side by starting
mysqld with the
--local-infile=0 option.
For the mysql command-line client, enable
LOAD DATA
LOCAL by specifying the
--local-infile[=1] option, or
disable it with the
--local-infile=0 option. For
mysqlimport, local data file loading is off
by default; enable it with the
--local or
-L option. In any case, successful use of a
local load operation requires that the server is enabled to
allow it.
If you use LOAD
DATA LOCAL in Perl scripts or other programs that
read the [client] group from option files,
you can add the local-infile=1 option to
that group. However, to keep this from causing problems for
programs that do not understand
local-infile, specify it using the
loose- prefix:
[client]
loose-local-infile=1
If LOAD DATA
LOCAL is disabled, either in the server or the
client, a client that attempts to issue such a statement
receives the following error message:
ERROR 1148: The used command is not allowed with this MySQL version
MySQL Enterprise
Security advisors notify subscribers to the MySQL Enterprise
Monitor whenever a server is started with the
--local-infile option enabled.
For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
5.3.5. How to Run MySQL as a Normal User
On Windows, you can run the server as a Windows service using a
normal user account.
On Unix, the MySQL server mysqld can be started
and run by any user. However, you should avoid running the server
as the Unix root user for security reasons. To
change mysqld to run as a normal unprivileged
Unix user user_name, you must do the
following:
Stop the server if it is running (use mysqladmin
shutdown).
Change the database directories and files so that
user_name has privileges to read
and write files in them (you might need to do this as the Unix
root user):
shell> chown -R user_name /path/to/mysql/datadir
If you do not do this, the server will not be able to access
databases or tables when it runs as
user_name.
If directories or files within the MySQL data directory are
symbolic links, chown -R might not follow
symbolic links for you. If it does not, you will also need to
follow those links and change the directories and files they
point to.
Start the server as user user_name.
Another alternative is to start mysqld as
the Unix root user and use the
--user=user_name
option. mysqld starts up, then switches to
run as the Unix user user_name
before accepting any connections.
To start the server as the given user automatically at system
startup time, specify the user name by adding a
user option to the
[mysqld] group of the
/etc/my.cnf option file or the
my.cnf option file in the server's data
directory. For example:
[mysqld]
user=user_name
If your Unix machine itself isn't secured, you should assign
passwords to the MySQL root accounts in the
grant tables. Otherwise, any user with a login account on that
machine can run the mysql client with a
--user=root option and perform any
operation. (It is a good idea to assign passwords to MySQL
accounts in any case, but especially so when other login accounts
exist on the server host.) See
Section 2.17, “Post-Installation Setup and Testing”.
5.4. The MySQL Access Privilege System
The primary function of the MySQL privilege system is to
authenticate a user who connects from a given host and to associate
that user with privileges on a database such as
SELECT,
INSERT,
UPDATE, and
DELETE. Additional functionality
includes the ability to have anonymous users and to grant privileges
for MySQL-specific functions such as
LOAD DATA
INFILE and administrative operations.
There are some things that you cannot do with the MySQL privilege
system:
You cannot explicitly specify that a given user should be denied
access. That is, you cannot explicitly match a user and then
refuse the connection.
You cannot specify that a user has privileges to create or drop
tables in a database but not to create or drop the database
itself.
A password applies globally to an account. You cannot associate
a password with a specific object such as a database, table, or
routine.
The user interface to the MySQL privilege system consists of SQL
statements such as CREATE USER,
GRANT, and
REVOKE. See
Section 12.5.1, “Account Management Statements”.
Internally, the server stores privilege information in the grant
tables of the mysql database (that is, in the
database named mysql). The MySQL server reads the
contents of these tables into memory when it starts and bases
access-control decisions on the in-memory copies of the grant
tables.
The MySQL privilege system ensures that all users may perform only
the operations allowed to them. As a user, when you connect to a
MySQL server, your identity is determined by the host from
which you connect and the user name you
specify. When you issue requests after connecting, the
system grants privileges according to your identity and
what you want to do.
MySQL considers both your host name and user name in identifying you
because there is no reason to assume that a given user name belongs
to the same person on all hosts. For example, the user
joe who connects from
office.example.com need not be the same person as
the user joe who connects from
home.example.com. MySQL handles this by allowing
you to distinguish users on different hosts that happen to have the
same name: You can grant one set of privileges for connections by
joe from office.example.com,
and a different set of privileges for connections by
joe from home.example.com. To
see what privileges a given account has, use the
SHOW GRANTS statement. For example:
SHOW GRANTS FOR 'joe'@'office.example.com';
SHOW GRANTS FOR 'joe'@'home.example.com';
MySQL access control involves two stages when you run a client
program that connects to the server:
Stage 1: The server accepts or
rejects the connection based on your identity and whether you can
verify your identity by supplying the correct password.
Stage 2: Assuming that you can
connect, the server checks each statement you issue to determine
whether you have sufficient privileges to perform it. For example,
if you try to select rows from a table in a database or drop a table
from the database, the server verifies that you have the
SELECT privilege for the table or the
DROP privilege for the database.
For a more detailed description of what happens during each stage,
see Section 5.4.4, “Access Control, Stage 1: Connection Verification”, and
Section 5.4.5, “Access Control, Stage 2: Request Verification”.
If your privileges are changed (either by yourself or someone else)
while you are connected, those changes do not necessarily take
effect immediately for the next statement that you issue. For
details about the conditions under which the server reloads the
grant tables, see Section 5.4.6, “When Privilege Changes Take Effect”.
For general security-related advice, see Section 5.3, “General Security Issues”.
For help in diagnosing privilege-related problems, see
Section 5.4.7, “Causes of Access-Denied Errors”.
5.4.1. Privileges Provided by MySQL
MySQL provides privileges that apply in different contexts and at
different levels of operation:
Administrative privileges enable users to manage operation of
the MySQL server. These privileges are global because they are
not specific to a particular database.
Database privileges apply to a database and to all objects
within it. These privileges can be granted for specific
databases, or globally so that they apply to all databases.
Privileges for database objects such as tables, indexes,
views, and stored routines can be granted for specific objects
within a database, for all objects of a given type within a
database (for example, all tables in a database), or globally
for all objects of a given type in all databases).
Information about account privileges is stored in the
user, db,
host, tables_priv,
columns_priv, and procs_priv
tables in the mysql database (see
Section 5.4.2, “Privilege System Grant Tables”). The MySQL server reads
the contents of these tables into memory when it starts and
reloads them under the circumstances indicated in
Section 5.4.6, “When Privilege Changes Take Effect”. Access-control decisions are
based on the in-memory copies of the grant tables.
Some releases of MySQL introduce changes to the structure of the
grant tables to add new access privileges or features. Whenever
you update to a new version of MySQL, you should update your grant
tables to make sure that they have the current structure so that
you can take advantage of any new capabilities. See
Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
The following table shows the privilege names used at the SQL
level in the GRANT and
REVOKE statements, along with the
column name associated with each privilege in the grant tables and
the context in which the privilege applies.
The following list provides a general description of each
privilege available in MySQL. Particular SQL statements might have
more specific privilege requirements than indicated here. If so,
the description for the statement in question provides the
details.
The ALL or
ALL PRIVILEGES
privilege specifier is shorthand. It stands for “all
privileges available at a given privilege level”
(except GRANT OPTION). For
example, granting ALL at the
global or table level grants all global privileges or all
table-level privileges.
The ALTER privilege enables use
of ALTER TABLE to change the
structure of or rename tables. (ALTER
TABLE also requires the
INSERT and
CREATE privileges.)
MySQL Enterprise
In some circumstances, the
ALTER privilege is entirely
unnecessary — on slaves where there are no
nonreplicated tables, for instance. The MySQL Enterprise
Monitor notifies subscribers when accounts have
inappropriate privileges. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
The ALTER ROUTINE privilege is
needed to alter or drop stored routines (procedures and
functions). This privilege was added in MySQL 5.0.3.
The CREATE privilege enables
creation of new databases and tables.
The CREATE ROUTINE privilege is
needed to create stored routines (procedures and functions).
This privilege was added in MySQL 5.0.3.
The CREATE TEMPORARY TABLES
privilege enables the use of the keyword
TEMPORARY in CREATE
TABLE statements.
The CREATE USER privilege
enables use of CREATE USER,
DROP USER,
RENAME USER, and
REVOKE ALL
PRIVILEGES. This privilege was added in MySQL 5.0.3.
The CREATE VIEW privilege
enables use of CREATE VIEW.
This privilege was added in MySQL 5.0.1.
The DELETE privilege enables
rows to be deleted from tables in a database.
The DROP privilege enables you
to drop (remove) existing databases and tables. If
you grant the DROP privilege
for the mysql database to a user, that user
can drop the database in which the MySQL access privileges are
stored.
The EXECUTE privilege is
required to execute stored routines (procedures and
functions). This privilege was added in MySQL 5.0.0 but did
not become operational until MySQL 5.0.3.
The FILE privilege gives you
permission to read and write files on the server host using
the LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE statements and the
LOAD_FILE() function. A user
who has the FILE privilege can
read any file on the server host that is either world-readable
or readable by the MySQL server. (This implies the user can
read any file in any database directory, because the server
can access any of those files.) The
FILE privilege also enables the
user to create new files in any directory where the MySQL
server has write access. As a security measure, the server
will not overwrite existing files.
The GRANT OPTION privilege
enables you to give to other users or remove from other users
those privileges that you yourself possess.
The INDEX privilege enables you
to create or drop (remove) indexes.
INDEX applies to existing
tables. If you have the CREATE
privilege for a table, you can include index definitions in
the CREATE TABLE statement.
The INSERT privilege enables
rows to be inserted into tables in a database.
INSERT is also required for the
ANALYZE TABLE,
OPTIMIZE TABLE, and
REPAIR TABLE table-maintenance
statements.
The LOCK TABLES privilege
enables the use of explicit LOCK
TABLES statements to lock tables for which you have
the SELECT privilege. This
includes the use of write locks, which prevents other sessions
from reading the locked table.
The PROCESS privilege pertains
to display of information about the threads executing within
the server (that is, information about the statements being
executed by sessions). The privilege enables use of
SHOW PROCESSLIST or
mysqladmin processlist to see threads
belonging to other accounts; you can always see your own
threads.
The REFERENCES privilege
currently is unused.
The RELOAD privilege enables
use of the FLUSH statement. It
also enables mysqladmin commands that are
equivalent to FLUSH operations:
flush-hosts, flush-logs,
flush-privileges,
flush-status,
flush-tables,
flush-threads, refresh,
and reload.
The reload command tells the server to
reload the grant tables into memory.
flush-privileges is a synonym for
reload. The refresh
command closes and reopens the log files and flushes all
tables. The other
flush-xxx
commands perform functions similar to
refresh, but are more specific and may be
preferable in some instances. For example, if you want to
flush just the log files, flush-logs is a
better choice than refresh.
The REPLICATION CLIENT
privilege enables the use of SHOW MASTER
STATUS and SHOW SLAVE
STATUS.
The REPLICATION SLAVE privilege
should be granted to accounts that are used by slave servers
to connect to the current server as their master. Without this
privilege, the slave cannot request updates that have been
made to databases on the master server.
The SELECT privilege enables
you to select rows from tables in a database.
SELECT statements require the
SELECT privilege only if they
actually retrieve rows from a table. Some
SELECT statements do not access
tables and can be executed without permission for any
database. For example, you can use
SELECT as a simple calculator
to evaluate expressions that make no reference to tables:
SELECT 1+1;
SELECT PI()*2;
The SELECT privilege is also
needed for other statements that read column values. For
example, SELECT is needed for
columns referenced on the right hand side of
col_name=expr
assignment in UPDATE statements
or for columns named in the WHERE clause of
DELETE or
UPDATE statements.
The SHOW DATABASES privilege
enables the account to see database names by issuing the
SHOW DATABASE statement. Accounts that do
not have this privilege see only databases for which they have
some privileges, and cannot use the statement at all if the
server was started with the
--skip-show-database option.
Note that any global privilege is a
privilege for the database.
MySQL Enterprise
The SHOW DATABASES privilege
should be granted only to users who need to see all the
databases on a MySQL server. Subscribers to the MySQL
Enterprise Monitor are alerted when servers are started
without the
--skip-show-database option.
For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
The SHOW VIEW privilege enables
use of SHOW CREATE VIEW. This
privilege was added in MySQL 5.0.1.
The SHUTDOWN privilege enables
use of the mysqladmin shutdown command.
There is no corresponding SQL statement.
The SUPER privilege enables an
account to use CHANGE MASTER
TO, KILL or
mysqladmin kill to kill threads belonging
to other accounts (you can always kill your own threads),
PURGE BINARY LOGS,
configuration changes via
SET
GLOBAL to modify global system variables, the
mysqladmin debug command, enabling or
disabling logging, performing updates even if the
read_only system variable is
enabled, starting and stopping replication on slave servers,
and allows you to connect (once) even if the connection limit
controlled by the
max_connections system
variable is reached.
To create or alter stored routines if binary logging is
enabled, you may also need the
SUPER privilege, as described
in Section 18.5, “Binary Logging of Stored Programs”.
The UPDATE privilege enables
rows to be updated in tables in a database.
The USAGE privilege specifier
stands for “no privileges.” It is used at the
global level with GRANT to
modify account attributes such as resource limits or SSL
characteristics without affecting existing account privileges.
It is a good idea to grant to an account only those privileges
that it needs. You should exercise particular caution in granting
the FILE and administrative
privileges:
The FILE privilege can be
abused to read into a database table any files that the MySQL
server can read on the server host. This includes all
world-readable files and files in the server's data directory.
The table can then be accessed using
SELECT to transfer its contents
to the client host.
The GRANT OPTION privilege
enables users to give their privileges to other users. Two
users that have different privileges and with the
GRANT OPTION privilege are able
to combine privileges.
The ALTER privilege may be used
to subvert the privilege system by renaming tables.
The SHUTDOWN privilege can be
abused to deny service to other users entirely by terminating
the server.
The PROCESS privilege can be
used to view the plain text of currently executing statements,
including statements that set or change passwords.
The SUPER privilege can be used
to terminate other sessions or change how the server operates.
Privileges granted for the mysql database
itself can be used to change passwords and other access
privilege information. Passwords are stored encrypted, so a
malicious user cannot simply read them to know the plain text
password. However, a user with write access to the
user table Password
column can change an account's password, and then connect to
the MySQL server using that account.
MySQL Enterprise
Accounts with unnecessary global privileges constitute a
security risk. Subscribers to the MySQL Enterprise Monitor
are automatically alerted to the existence of such accounts.
For detailed information, see
http://www.mysql.com/products/enterprise/advisors.html.
5.4.2. Privilege System Grant Tables
Normally, you manipulate the contents of the grant tables
indirectly by using statements such as
GRANT and
REVOKE to set up accounts and
control the privileges available to each one. See
Section 12.5.1, “Account Management Statements”. The discussion here
describes the underlying structure of the grant tables and how the
server uses their contents when interacting with clients.
Each grant table contains scope columns and privilege columns:
Scope columns determine the scope of each row (entry) in the
tables; that is, the context in which the row applies. For
example, a user table row with
Host and User values of
'thomas.loc.gov' and
'bob' would be used for authenticating
connections made to the server from the host
thomas.loc.gov by a client that specifies a
user name of bob. Similarly, a
db table row with Host,
User, and Db column
values of 'thomas.loc.gov',
'bob' and 'reports'
would be used when bob connects from the
host thomas.loc.gov to access the
reports database. The
tables_priv and
columns_priv tables contain scope columns
indicating tables or table/column combinations to which each
row applies. The procs_priv scope columns
indicate the stored routine to which each row applies.
Privilege columns indicate which privileges are granted by a
table row; that is, what operations can be performed. The
server combines the information in the various grant tables to
form a complete description of a user's privileges.
Section 5.4.5, “Access Control, Stage 2: Request Verification”, describes the rules that are
used to do this.
The server uses the grant tables in the following manner:
The user table scope columns determine
whether to reject or allow incoming connections. For allowed
connections, any privileges granted in the
user table indicate the user's global
(superuser) privileges. Any privilege granted in this table
applies to all databases on the server.
Note
Because any global privilege is considered a privilege for
all databases, any global privilege enables a user to see
all database names with SHOW
DATABASES or by examining the
SCHEMATA table of
INFORMATION_SCHEMA.
The db table scope columns determine which
users can access which databases from which hosts. The
privilege columns determine which operations are allowed. A
privilege granted at the database level applies to the
database and to all objects in the database, such as tables
and stored programs.
The host table is used in conjunction with
the db table when you want a given
db table row to apply to several hosts. For
example, if you want a user to be able to use a database from
several hosts in your network, leave the
Host value empty in the user's
db table row, then populate the
host table with a row for each of those
hosts. This mechanism is described more detail in
Section 5.4.5, “Access Control, Stage 2: Request Verification”.
Note
The host table must be modified directly
with statements such as
INSERT,
UPDATE, and
DELETE. It is not affected by
statements such as GRANT and
REVOKE that modify the grant
tables indirectly. Most MySQL installations need not use
this table at all.
The tables_priv and
columns_priv tables are similar to the
db table, but are more fine-grained: They
apply at the table and column levels rather than at the
database level. A privilege granted at the table level applies
to the table and to all its columns. A privilege granted at
the column level applies only to a specific column.
The procs_priv table applies to stored
routines. A privilege granted at the routine level applies
only to a single routine.
The server uses the user,
db, and host tables in the
mysql database at both the first and second
stages of access control (see Section 5.4, “The MySQL Access Privilege System”).
The columns in the user and
db tables are shown here. The
host table is similar to the
db table but has a specialized use as described
in Section 5.4.5, “Access Control, Stage 2: Request Verification”.
Execute_priv was present in MySQL 5.0.0, but
did not become operational until MySQL 5.0.3.
The Create_view_priv and
Show_view_priv columns were added in MySQL
5.0.1.
The Create_routine_priv,
Alter_routine_priv, and
max_user_connections columns were
added in MySQL 5.0.3.
During the second stage of access control, the server performs
request verification to make sure that each client has sufficient
privileges for each request that it issues. In addition to the
user, db, and
host grant tables, the server may also consult
the tables_priv and
columns_priv tables for requests that involve
tables. The latter tables provide finer privilege control at the
table and column levels. They have the columns shown in the
following table.
The Timestamp and Grantor
columns currently are unused and are discussed no further here.
For verification of requests that involve stored routines, the
server may consult the procs_priv table, which
has the columns shown in the following table.
The procs_priv table exists as of MySQL 5.0.3.
The Routine_type column was added in MySQL
5.0.6. It is an ENUM column with
values of 'FUNCTION' or
'PROCEDURE' to indicate the type of routine the
row refers to. This column enables privileges to be granted
separately for a function and a procedure with the same name.
The Timestamp and Grantor
columns currently are unused and are discussed no further here.
Scope columns in the grant tables contain strings. They are
declared as shown here; the default value for each is the empty
string.
For access-checking purposes, comparisons of
User, Password,
Db, and Table_name values
are case sensitive. Comparisons of Host,
Column_name, and
Routine_name values are not case sensitive.
In the user, db, and
host tables, each privilege is listed in a
separate column that is declared as ENUM('N','Y') DEFAULT
'N'. In other words, each privilege can be disabled or
enabled, with the default being disabled.
In the tables_priv,
columns_priv, and procs_priv
tables, the privilege columns are declared as
SET columns. Values in these
columns can contain any combination of the privileges controlled
by the table. Only those privileges listed in the column value are
enabled.
Administrative privileges (such as
RELOAD or
SHUTDOWN) are specified only in the
user table. Administrative operations are
operations on the server itself and are not database-specific, so
there is no reason to list these privileges in the other grant
tables. Consequently, to determine whether you can perform an
administrative operation, the server need consult only the
user table.
The FILE privilege also is
specified only in the user table. It is not an
administrative privilege as such, but your ability to read or
write files on the server host is independent of the database you
are accessing.
The mysqld server reads the contents of the
grant tables into memory when it starts. You can tell it to reload
the tables by issuing a
FLUSH PRIVILEGES
statement or executing a mysqladmin
flush-privileges or mysqladmin reload
command. Changes to the grant tables take effect as indicated in
Section 5.4.6, “When Privilege Changes Take Effect”.
When you modify an account's privileges, it is a good idea to
verify that the changes set up privileges the way you want. To
check the privileges for a given account, use the
SHOW GRANTS statement (see
Section 12.5.5.17, “SHOW GRANTS Syntax”). For example, to determine the
privileges that are granted to an account with user name and host
name values of bob and
pc84.example.com, use this statement:
SHOW GRANTS FOR 'bob'@'pc84.example.com';
5.4.3. Specifying Account Names
MySQL account names consist of a user name and a host name. This
enables creation of accounts for users with the same name who can
connect from different hosts. This section describes how to write
account names, including special values and wildcard rules.
Within SQL statements such as CREATE
USER, GRANT, and
SET PASSWORD, account names are
written using the following rules:
Syntax for account names is
'user_name'@'host_name'.
An account name consisting only of a user name is equivalent
to
'user_name'@'%'.
For example, 'me' is equivalent to
'me'@'%'.
The user name and host name need not be quoted if they are
legal as unquoted identifiers. Quotes are necessary to specify
a user_name string containing
special characters (such as
“-”), or a
host_name string containing special
characters or wildcard characters (such as
“%”); for example,
'test-user'@'%.com'.
Quote user names and host names as identifiers or as strings,
using either backticks (“`”),
single quotes (“'”), or double
quotes (“"”).
The user name and host name parts, if quoted, must be quoted
separately. That is, write
'me'@'localhost', not
'me@localhost'; the latter is interpreted
as 'me@localhost'@'%'.
Account names are stored in grant tables using separate columns
for the user name and host name parts:
The user table contains one row for each
account. The User and
Host columns store the user name and host
name. Another column, Password, stores the
account password. This table also indicates which global
privileges the account has.
Other grant tables indicate privileges an account has for
databases and objects within databases. These tables have
User and Host columns to
store the account name. Each row in these tables associates
with the account in the user table that has
the same User and Host
values.
For additional detail about grant table structure, see
Section 5.4.2, “Privilege System Grant Tables”.
User names and host names have certain special values or wildcard
conventions, as described following.
A user name is either a nonblank value that literally matches the
user name for incoming connection attempts, or a blank value
(empty string) that matches any user name. An account with a blank
user name is an anonymous user. To specify an anonymous user in
SQL statements, use a quoted empty user name part, such as
''@'localhost'.
The host part of an account name can take many forms, and
wildcards are allowed:
A host value can be a host name or an IP number.
'localhost' indicates the local host.
'127.0.0.1' indicates the loopback
interface.
You can use the wildcard characters
“%” and
“_” in host values. These have
the same meaning as for pattern-matching operations performed
with the LIKE operator. For
example, a host value of '%' matches any
host name, whereas a value of '%.mysql.com'
matches any host in the mysql.com domain.
'192.168.1.%' matches any host in the
192.168.1 class C network.
Because you can use IP wildcard values in host values (for
example, '192.168.1.%' to match every host
on a subnet), someone could try to exploit this capability by
naming a host 192.168.1.somewhere.com. To
foil such attempts, MySQL disallows matching on host names
that start with digits and a dot. Thus, if you have a host
named something like 1.2.example.com, its
name never matches the host part of account names. An IP
wildcard value can match only IP numbers, not host names.
MySQL Enterprise
An overly broad host specifier such as
“%” constitutes a security
risk. The MySQL Enterprise Monitor provides safeguards
against this kind of vulnerability. For more information,
see http://www.mysql.com/products/enterprise/advisors.html.
For host values specified as IP numbers, you can specify a
netmask indicating how many address bits to use for the
network number. The syntax is
host_ip/netmask CREATE USER 'david'@'192.58.197.0/255.255.255.0';
This enables david to connect from any
client host having an IP number
client_ip for which the following
condition is true:
client_ip & netmask = host_ip
That is, for the CREATE USER
statement just shown:
client_ip & 255.255.255.0 = 192.58.197.0
IP numbers that satisfy this condition and can connect to the
MySQL server are those in the range from
192.58.197.0 to
192.58.197.255.
The netmask can only be used to tell the server to use 8, 16,
24, or 32 bits of the address. Examples:
192.0.0.0/255.0.0.0: anything on the
192 class A network
192.168.0.0/255.255.0.0: anything on
the 192.168 class B network
192.168.1.0/255.255.255.0: anything on
the 192.168.1 class C network
192.168.1.1: only this specific IP
The following netmask (28 bits) will not work:
192.168.0.1/255.255.255.240
5.4.4. Access Control, Stage 1: Connection Verification
When you attempt to connect to a MySQL server, the server accepts
or rejects the connection based on your identity and whether you
can verify your identity by supplying the correct password. If
not, the server denies access to you completely. Otherwise, the
server accepts the connection, and then enters Stage 2 and waits
for requests.
Your identity is based on two pieces of information:
Identity checking is performed using the three
user table scope columns
(Host, User, and
Password). The server accepts the connection
only if the Host and User
columns in some user table row match the client
host name and user name and the client supplies the password
specified in that row. The rules for allowable
Host and User values are
given in Section 5.4.3, “Specifying Account Names”.
If the User column value is nonblank, the user
name in an incoming connection must match exactly. If the
User value is blank, it matches any user name.
If the user table row that matches an incoming
connection has a blank user name, the user is considered to be an
anonymous user with no name, not a user with the name that the
client actually specified. This means that a blank user name is
used for all further access checking for the duration of the
connection (that is, during Stage 2).
The Password column can be blank. This is not a
wildcard and does not mean that any password matches. It means
that the user must connect without specifying a password.
Nonblank Password values in the
user table represent encrypted passwords. MySQL
does not store passwords in plaintext form for anyone to see.
Rather, the password supplied by a user who is attempting to
connect is encrypted (using the
PASSWORD() function). The encrypted
password then is used during the connection process when checking
whether the password is correct. (This is done without the
encrypted password ever traveling over the connection.) See
Section 5.5.1, “User Names and Passwords”.
From MySQL's point of view, the encrypted password is the
real password, so you should never give
anyone access to it. In particular, do not give
nonadministrative users read access to tables in the
mysql database.
The following table shows how various combinations of
Host and User values in the
user table apply to incoming connections.
It is possible for the client host name and user name of an
incoming connection to match more than one row in the
user table. The preceding set of examples
demonstrates this: Several of the entries shown match a connection
from thomas.loc.gov by fred.
When multiple matches are possible, the server must determine
which of them to use. It resolves this issue as follows:
Whenever the server reads the user table
into memory, it sorts the rows.
When a client attempts to connect, the server looks through
the rows in sorted order.
The server uses the first row that matches the client host
name and user name.
To see how this works, suppose that the user
table looks like this:
+-----------+----------+-
| Host | User | ...
+-----------+----------+-
| % | root | ...
| % | jeffrey | ...
| localhost | root | ...
| localhost | | ...
+-----------+----------+-
When the server reads the table into memory, it orders the rows
with the most-specific Host values first.
Literal host names and IP numbers are the most specific. (The
specificity if a literal IP number is not affected by whether it
has a netmask, so 192.168.1.13 and
192.168.1.0/255.255.255.0 are considered
equally specific.) The pattern '%' means
“any host” and is least specific. Rows with the same
Host value are ordered with the most-specific
User values first (a blank
User value means “any user” and is
least specific). For the user table just shown,
the result after sorting looks like this:
+-----------+----------+-
| Host | User | ...
+-----------+----------+-
| localhost | root | ...
| localhost | | ...
| % | jeffrey | ...
| % | root | ...
+-----------+----------+-
When a client attempts to connect, the server looks through the
sorted rows and uses the first match found. For a connection from
localhost by jeffrey, two of
the rows from the table match: the one with
Host and User values of
'localhost' and '', and the
one with values of '%' and
'jeffrey'. The 'localhost'
row appears first in sorted order, so that is the one the server
uses.
Here is another example. Suppose that the user
table looks like this:
+----------------+----------+-
| Host | User | ...
+----------------+----------+-
| % | jeffrey | ...
| thomas.loc.gov | | ...
+----------------+----------+-
The sorted table looks like this:
+----------------+----------+-
| Host | User | ...
+----------------+----------+-
| thomas.loc.gov | | ...
| % | jeffrey | ...
+----------------+----------+-
A connection by jeffrey from
thomas.loc.gov is matched by the first row,
whereas a connection by jeffrey from any host
is matched by the second.
Note
It is a common misconception to think that, for a given user
name, all rows that explicitly name that user are used first
when the server attempts to find a match for the connection.
This is not true. The preceding example illustrates this, where
a connection from thomas.loc.gov by
jeffrey is first matched not by the row
containing 'jeffrey' as the
User column value, but by the row with no
user name. As a result, jeffrey is
authenticated as an anonymous user, even though he specified a
user name when connecting.
If you are able to connect to the server, but your privileges are
not what you expect, you probably are being authenticated as some
other account. To find out what account the server used to
authenticate you, use the
CURRENT_USER() function. (See
Section 11.10.3, “Information Functions”.) It returns a value in
user_name@host_nameUser and
Host values from the matching
user table row. Suppose that
jeffrey connects and issues the following
query:
mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost |
+----------------+
The result shown here indicates that the matching
user table row had a blank
User column value. In other words, the server
is treating jeffrey as an anonymous user.
Another way to diagnose authentication problems is to print out
the user table and sort it by hand to see where
the first match is being made.
5.4.5. Access Control, Stage 2: Request Verification
After you establish a connection, the server enters Stage 2 of
access control. For each request that you issue via that
connection, the server determines what operation you want to
perform, then checks whether you have sufficient privileges to do
so. This is where the privilege columns in the grant tables come
into play. These privileges can come from any of the
user, db,
host, tables_priv,
columns_priv, or procs_priv
tables. (You may find it helpful to refer to
Section 5.4.2, “Privilege System Grant Tables”, which lists the columns
present in each of the grant tables.)
The user table grants privileges that are
assigned to you on a global basis and that apply no matter what
the default database is. For example, if the
user table grants you the
DELETE privilege, you can delete
rows from any table in any database on the server host! In other
words, user table privileges are superuser
privileges. It is wise to grant privileges in the
user table only to superusers such as database
administrators. For other users, you should leave all privileges
in the user table set to 'N'
and grant privileges at more specific levels only. You can grant
privileges for particular databases, tables, columns, or routines.
The db and host tables grant
database-specific privileges. Values in the scope columns of these
tables can take the following forms:
A blank User value in the
db table matches the anonymous user. A
nonblank value matches literally; there are no wildcards in
user names.
The wildcard characters “%”
and “_” can be used in the
Host and Db columns of
either table. These have the same meaning as for
pattern-matching operations performed with the
LIKE operator. If you want to use
either character literally when granting privileges, you must
escape it with a backslash. For example, to include the
underscore character (“_”) as
part of a database name, specify it as
“\_” in the
GRANT statement.
A '%' Host value in the
db table means “any host.” A
blank Host value in the
db table means “consult the
host table for further information”
(a process that is described later in this section).
A '%' or blank Host
value in the host table means “any
host.”
A '%' or blank Db value
in either table means “any database.”
The server reads the db and
host tables into memory and sorts them at the
same time that it reads the user table. The
server sorts the db table based on the
Host, Db, and
User scope columns, and sorts the
host table based on the Host
and Db scope columns. As with the
user table, sorting puts the most-specific
values first and least-specific values last, and when the server
looks for matching entries, it uses the first match that it finds.
The tables_priv,
columns_priv, and procs_priv
tables grant table-specific, column-specific, and routine-specific
privileges. Values in the scope columns of these tables can take
the following forms:
The wildcard characters “%”
and “_” can be used in the
Host column. These have the same meaning as
for pattern-matching operations performed with the
LIKE operator.
A '%' or blank Host
value means “any host.”
The Db, Table_name,
Column_name, and
Routine_name columns cannot contain
wildcards or be blank.
The server sorts the tables_priv,
columns_priv, and procs_priv
tables based on the Host,
Db, and User columns. This
is similar to db table sorting, but simpler
because only the Host column can contain
wildcards.
The server uses the sorted tables to verify each request that it
receives. For requests that require administrative privileges such
as SHUTDOWN or
RELOAD, the server checks only the
user table row because that is the only table
that specifies administrative privileges. The server grants access
if the row allows the requested operation and denies access
otherwise. For example, if you want to execute mysqladmin
shutdown but your user table row
doesn't grant the SHUTDOWN
privilege to you, the server denies access without even checking
the db or host tables. (They
contain no Shutdown_priv column, so there is no
need to do so.)
For database-related requests
(INSERT,
UPDATE, and so on), the server
first checks the user's global (superuser) privileges by looking
in the user table row. If the row allows the
requested operation, access is granted. If the global privileges
in the user table are insufficient, the server
determines the user's database-specific privileges by checking the
db and host tables:
The server looks in the db table for a
match on the Host, Db,
and User columns. The
Host and User columns
are matched to the connecting user's host name and MySQL user
name. The Db column is matched to the
database that the user wants to access. If there is no row for
the Host and User,
access is denied.
If there is a matching db table row and its
Host column is not blank, that row defines
the user's database-specific privileges.
If the matching db table row's
Host column is blank, it signifies that the
host table enumerates which hosts should be
allowed access to the database. In this case, a further lookup
is done in the host table to find a match
on the Host and Db
columns. If no host table row matches,
access is denied. If there is a match, the user's
database-specific privileges are computed as the intersection
(not the union!) of the privileges in the
db and host table
entries; that is, the privileges that are
'Y' in both entries. (This way you can
grant general privileges in the db table
row and then selectively restrict them on a host-by-host basis
using the host table entries.)
After determining the database-specific privileges granted by the
db and host table entries,
the server adds them to the global privileges granted by the
user table. If the result allows the requested
operation, access is granted. Otherwise, the server successively
checks the user's table and column privileges in the
tables_priv and columns_priv
tables, adds those to the user's privileges, and allows or denies
access based on the result. For stored-routine operations, the
server uses the procs_priv table rather than
tables_priv and
columns_priv.
Expressed in boolean terms, the preceding description of how a
user's privileges are calculated may be summarized like this:
global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges
OR routine privileges
It may not be apparent why, if the global user
row privileges are initially found to be insufficient for the
requested operation, the server adds those privileges to the
database, table, and column privileges later. The reason is that a
request might require more than one type of privilege. For
example, if you execute an
INSERT INTO ...
SELECT statement, you need both the
INSERT and the
SELECT privileges. Your privileges
might be such that the user table row grants
one privilege and the db table row grants the
other. In this case, you have the necessary privileges to perform
the request, but the server cannot tell that from either table by
itself; the privileges granted by the entries in both tables must
be combined.
The host table is not affected by the
GRANT or
REVOKE statements, so it is unused
in most MySQL installations. If you modify it directly, you can
use it for some specialized purposes, such as to maintain a list
of secure servers on the local network that are granted all
privileges.
You can also use the host table to indicate
hosts that are not secure. Suppose that you
have a machine public.your.domain that is
located in a public area that you do not consider secure. You can
enable access to all hosts on your network except that machine by
using host table entries like this:
+--------------------+----+-
| Host | Db | ...
+--------------------+----+-
| public.your.domain | % | ... (all privileges set to 'N')
| %.your.domain | % | ... (all privileges set to 'Y')
+--------------------+----+-
5.4.6. When Privilege Changes Take Effect
When mysqld starts, it reads all grant table
contents into memory. The in-memory tables become effective for
access control at that point.
If you modify the grant tables indirectly using account-management
statements such as GRANT,
REVOKE, or SET
PASSWORD, the server notices these changes and loads the
grant tables into memory again immediately.
If you modify the grant tables directly using statements such as
INSERT,
UPDATE, or
DELETE, your changes have no effect
on privilege checking until you either restart the server or tell
it to reload the tables. If you change the grant tables directly
but forget to reload them, your changes have no
effect until you restart the server. This may leave you
wondering why your changes do not seem to make any difference!
To tell the sever to reload the grant tables, perform a
flush-privileges operation. This can be done by issuing a
FLUSH PRIVILEGES
statement or by executing a mysqladmin
flush-privileges or mysqladmin reload
command.
When the server reloads the grant tables, privileges for each
existing client connection are affected as follows:
Table and column privilege changes take effect with the
client's next request.
Database privilege changes take effect the next time the
client executes a USE
db_name statement.
Note
Client applications may cache the database name; thus, this
effect may not be visible to them without actually changing
to a different database or flushing the privileges.
Global privileges and passwords are unaffected for a connected
client. These changes take effect only for subsequent
connections.
If the server is started with the
--skip-grant-tables option, it does
not read the grant tables or implement any access control. Anyone
can connect and do anything. To cause a server thus started to
read the tables and enable access checking, flush the privileges.
5.4.7. Causes of Access-Denied Errors
If you encounter problems when you try to connect to the MySQL
server, the following items describe some courses of action you
can take to correct the problem.
Make sure that the server is running. If it is not, clients
cannot connect to it. For example, if an attempt to connect to
the server fails with a message such as one of those
following, one cause might be that the server is not running:
shell> mysql
ERROR 2003: Can't connect to MySQL server on 'host_name' (111)
shell> mysql
ERROR 2002: Can't connect to local MySQL server through socket
'/tmp/mysql.sock' (111)
It might be that the server is running, but you are trying to
connect using a TCP/IP port, named pipe, or Unix socket file
different from the one on which the server is listening. To
correct this when you invoke a client program, specify a
--port option to indicate the
proper port number, or a
--socket option to indicate
the proper named pipe or Unix socket file. To find out where
the socket file is, you can use this command:
shell> netstat -ln | grep mysql
Make sure that the server has not been configured to ignore
network connections or (if you are attempting to connect
remotely) that it has not been configured to listen only
locally on its network interfaces. If the server was started
with --skip-networking, it will
not accept TCP/IP connections at all. If the server was
started with
--bind-address=127.0.0.1, it
will listen for TCP/IP connections only locally on the
loopback interface and will not accept remote connections.
Check to make sure that there is no firewall blocking access
to MySQL. Your firewall may be configured on the basis of the
application being executed, or the port number used by MySQL
for communication (3306 by default). Under Linux or Unix,
check your IP tables (or similar) configuration to ensure that
the port has not been blocked. Under Windows, applications
such as ZoneAlarm or the Windows XP personal firewall may need
to be configured not to block the MySQL port.
The grant tables must be properly set up so that the server
can use them for access control. For some distribution types
(such as binary distributions on Windows, or RPM distributions
on Linux), the installation process initializes the
mysql database containing the grant tables.
For distributions that do not do this, you must initialize the
grant tables manually by running the
mysql_install_db script. For details, see
Section 2.17.2, “Unix Post-Installation Procedures”.
To determine whether you need to initialize the grant tables,
look for a mysql directory under the data
directory. (The data directory normally is named
data or var and is
located under your MySQL installation directory.) Make sure
that you have a file named user.MYD in
the mysql database directory. If not,
execute the mysql_install_db script. After
running this script and starting the server, test the initial
privileges by executing this command:
shell> mysql -u root test
The server should let you connect without error.
After a fresh installation, you should connect to the server
and set up your users and their access permissions:
shell> mysql -u root mysql
The server should let you connect because the MySQL
root user has no password initially. That
is also a security risk, so setting the password for the
root accounts is something you should do
while you're setting up your other MySQL accounts. For
instructions on setting the initial passwords, see
Section 2.17.3, “Securing the Initial MySQL Accounts”.
MySQL Enterprise
The MySQL Enterprise Monitor enforces security-related best
practices. For example, subscribers are alerted whenever
there is any account without a password. For more
information, see
http://www.mysql.com/products/enterprise/advisors.html.
If you have updated an existing MySQL installation to a newer
version, did you run the mysql_upgrade
script? If not, do so. The structure of the grant tables
changes occasionally when new capabilities are added, so after
an upgrade you should always make sure that your tables have
the current structure. For instructions, see
Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
If a client program receives the following error message when
it tries to connect, it means that the server expects
passwords in a newer format than the client is capable of
generating:
shell> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client
For information on how to deal with this, see
Section 5.5.6.3, “Password Hashing in MySQL”, and
Section B.5.2.4, “Client does not support authentication protocol”.
Remember that client programs use connection parameters
specified in option files or environment variables. If a
client program seems to be sending incorrect default
connection parameters when you have not specified them on the
command line, check any applicable option files and your
environment. For example, if you get Access
denied when you run a client without any options,
make sure that you have not specified an old password in any
of your option files!
You can suppress the use of option files by a client program
by invoking it with the
--no-defaults option. For
example:
shell> mysqladmin --no-defaults -u root version
The option files that clients use are listed in
Section 4.2.3.3, “Using Option Files”. Environment variables are
listed in Section 2.20, “Environment Variables”.
If you get the following error, it means that you are using an
incorrect root password:
shell> mysqladmin -u root -pxxxx ver
Access denied for user 'root'@'localhost' (using password: YES)
If the preceding error occurs even when you have not specified
a password, it means that you have an incorrect password
listed in some option file. Try the
--no-defaults option as
described in the previous item.
For information on changing passwords, see
Section 5.5.5, “Assigning Account Passwords”.
If you have lost or forgotten the root
password, see Section B.5.4.1, “How to Reset the Root Password”.
If you change a password by using SET
PASSWORD, INSERT, or
UPDATE, you must encrypt the
password using the PASSWORD()
function. If you do not use
PASSWORD() for these
statements, the password will not work. For example, the
following statement assigns a password, but fails to encrypt
it, so the user is not able to connect afterward:
SET PASSWORD FOR 'abe'@'host_name' = 'eagle';
Instead, set the password like this:
SET PASSWORD FOR 'abe'@'host_name' = PASSWORD('eagle');
The PASSWORD() function is
unnecessary when you specify a password using the
GRANT or (beginning with MySQL
5.0.2) CREATE USER statements,
or the mysqladmin password command. Each of
those automatically uses
PASSWORD() to encrypt the
password. See Section 5.5.5, “Assigning Account Passwords”, and
Section 12.5.1.1, “CREATE USER Syntax”.
localhost is a synonym for your local host
name, and is also the default host to which clients try to
connect if you specify no host explicitly.
To avoid this problem on such systems, you can use a
--host=127.0.0.1 option to
name the server host explicitly. This will make a TCP/IP
connection to the local mysqld server. You
can also use TCP/IP by specifying a
--host option that uses the
actual host name of the local host. In this case, the host
name must be specified in a user table row
on the server host, even though you are running the client
program on the same host as the server.
The Access denied error message tells you
who you are trying to log in as, the client host from which
you are trying to connect, and whether you were using a
password. Normally, you should have one row in the
user table that exactly matches the host
name and user name that were given in the error message. For
example, if you get an error message that contains
using password: NO, it means that you tried
to log in without a password.
If you get an Access denied error when
trying to connect to the database with mysql -u
user_name, you may have a
problem with the user table. Check this by
executing mysql -u root mysql and issuing
this SQL statement:
SELECT * FROM user;
The result should include a row with the
Host and User columns
matching your client's host name and your MySQL user name.
If the following error occurs when you try to connect from a
host other than the one on which the MySQL server is running,
it means that there is no row in the user
table with a Host value that matches the
client host:
Host ... is not allowed to connect to this MySQL server
You can fix this by setting up an account for the combination
of client host name and user name that you are using when
trying to connect.
If you do not know the IP number or host name of the machine
from which you are connecting, you should put a row with
'%' as the Host column
value in the user table. After trying to
connect from the client machine, use a SELECT
USER() query to see how you really did connect. Then
change the '%' in the
user table row to the actual host name that
shows up in the log. Otherwise, your system is left insecure
because it allows connections from any host for the given user
name.
On Linux, another reason that this error might occur is that
you are using a binary MySQL version that is compiled with a
different version of the glibc library than
the one you are using. In this case, you should either upgrade
your operating system or glibc, or download
a source distribution of MySQL version and compile it
yourself. A source RPM is normally trivial to compile and
install, so this is not a big problem.
If you specify a host name when trying to connect, but get an
error message where the host name is not shown or is an IP
number, it means that the MySQL server got an error when
trying to resolve the IP number of the client host to a name:
shell> mysqladmin -u root -pxxxx -h some_hostname ver
Access denied for user 'root'@'' (using password: YES)
If you try to connect as root and get the
following error, it means that you do not have a row in the
user table with a User
column value of 'root' and that
mysqld cannot resolve the host name for
your client:
Access denied for user ''@'unknown'
These errors indicate a DNS problem. To fix it, execute
mysqladmin flush-hosts to reset the
internal DNS host name cache. See Section 7.5.11, “How MySQL Uses DNS”.
Some permanent solutions are:
Determine what is wrong with your DNS server and fix it.
Specify IP numbers rather than host names in the MySQL
grant tables.
Put an entry for the client machine name in
/etc/hosts on Unix or
\windows\hosts on Windows.
Start mysqld with the
--skip-name-resolve option.
Start mysqld with the
--skip-host-cache option.
On Unix, if you are running the server and the client on
the same machine, connect to localhost.
Unix connections to localhost use a
Unix socket file rather than TCP/IP.
On Windows, if you are running the server and the client
on the same machine and the server supports named pipe
connections, connect to the host name .
(period). Connections to . use a named
pipe rather than TCP/IP.
If mysql -u root test works but
mysql -h your_hostname -u
root test results in Access
denied (where
your_hostname is the actual host
name of the local host), you may not have the correct name for
your host in the user table. A common
problem here is that the Host value in the
user table row specifies an unqualified
host name, but your system's name resolution routines return a
fully qualified domain name (or vice versa). For example, if
you have an entry with host 'pluto' in the
user table, but your DNS tells MySQL that
your host name is 'pluto.example.com', the
entry does not work. Try adding an entry to the
user table that contains the IP number of
your host as the Host column value.
(Alternatively, you could add an entry to the
user table with a Host
value that contains a wildcard; for example,
'pluto.%'. However, use of
Host values ending with
“%” is
insecure and is not
recommended!)
If mysql -u user_name
test works but mysql -u
user_name
other_db does not, you
have not granted access to the given user for the database
named other_db.
If mysql -u
user_name works when
executed on the server host, but mysql -h
host_name -u
user_name does not work
when executed on a remote client host, you have not enabled
access to the server for the given user name from the remote
host.
If you cannot figure out why you get Access
denied, remove from the user
table all entries that have Host values
containing wildcards (entries that contain
'%' or '_' characters).
A very common error is to insert a new entry with
Host='%' and
User='some_user',
thinking that this allows you to specify
localhost to connect from the same machine.
The reason that this does not work is that the default
privileges include an entry with
Host='localhost' and
User=''. Because that
entry has a Host value
'localhost' that is more specific than
'%', it is used in preference to the new
entry when connecting from localhost! The
correct procedure is to insert a second entry with
Host='localhost' and
User='some_user',
or to delete the entry with
Host='localhost' and
User=''. After deleting
the entry, remember to issue a
FLUSH
PRIVILEGES statement to reload the grant tables. See
also Section 5.4.4, “Access Control, Stage 1: Connection Verification”.
If you are able to connect to the MySQL server, but get an
Access denied message whenever you issue a
SELECT ... INTO
OUTFILE or
LOAD DATA
INFILE statement, your entry in the
user table does not have the
FILE privilege enabled.
If you change the grant tables directly (for example, by using
INSERT,
UPDATE, or
DELETE statements) and your
changes seem to be ignored, remember that you must execute a
FLUSH
PRIVILEGES statement or a mysqladmin
flush-privileges command to cause the server to
reload the privilege tables. Otherwise, your changes have no
effect until the next time the server is restarted. Remember
that after you change the root password
with an UPDATE command, you
will not need to specify the new password until after you
flush the privileges, because the server will not know you've
changed the password yet!
If your privileges seem to have changed in the middle of a
session, it may be that a MySQL administrator has changed
them. Reloading the grant tables affects new client
connections, but it also affects existing connections as
indicated in Section 5.4.6, “When Privilege Changes Take Effect”.
If you have access problems with a Perl, PHP, Python, or ODBC
program, try to connect to the server with mysql -u
user_name
db_name or mysql
-u user_name
-pyour_pass
db_name. If you are able
to connect using the mysql client, the
problem lies with your program, not with the access
privileges. (There is no space between -p and
the password; you can also use the
--password=your_pass
syntax to specify the password. If you use the
-p or
--password option with no
password value, MySQL prompts you for the password.)
For testing purposes, start the mysqld
server with the
--skip-grant-tables option.
Then you can change the MySQL grant tables and use the
mysqlaccess script to check whether your
modifications have the desired effect. When you are satisfied
with your changes, execute mysqladmin
flush-privileges to tell the
mysqld server to reload the privileges.
This enables you to begin using the new grant table contents
without stopping and restarting the server.
If you get the following error, you may have a problem with
the db or host table:
Access to database denied
If the entry selected from the db table has
an empty value in the Host column, make
sure that there are one or more corresponding entries in the
host table specifying which hosts the
db table entry applies to. This problem
occurs infrequently because the host table
is rarely used.
If everything else fails, start the mysqld
server with a debugging option (for example,
--debug=d,general,query). This
prints host and user information about attempted connections,
as well as information about each command issued. See
MySQL
Internals: Porting.
If you have any other problems with the MySQL grant tables and
feel you must post the problem to the mailing list, always
provide a dump of the MySQL grant tables. You can dump the
tables with the mysqldump mysql command. To
file a bug report, see the instructions at
Section 1.7, “How to Report Bugs or Problems”. In some cases, you may need to
restart mysqld with
--skip-grant-tables to run
mysqldump.
5.5. MySQL User Account Management
This section describes how to set up accounts for clients of your
MySQL server. It discusses the following topics:
The meaning of account names and passwords as used in MySQL and
how that compares to names and passwords used by your operating
system
How to set up new accounts and remove existing accounts
How to change passwords
Guidelines for using passwords securely
How to use secure connections with SSL
See also Section 12.5.1, “Account Management Statements”, which describes
the syntax and use for all user-management SQL statements.
5.5.1. User Names and Passwords
A MySQL account is defined in terms of a user name and the client
host or hosts from which the user can connect to the server. The
account also has a password. There are several distinctions
between the way user names and passwords are used by MySQL and the
way they are used by your operating system:
User names, as used by MySQL for authentication purposes, have
nothing to do with user names (login names) as used by Windows
or Unix. On Unix, most MySQL clients by default try to log in
using the current Unix user name as the MySQL user name, but
that is for convenience only. The default can be overridden
easily, because client programs allow any user name to be
specified with a -u or
--user option. Because this means that anyone
can attempt to connect to the server using any user name, you
cannot make a database secure in any way unless all MySQL
accounts have passwords. Anyone who specifies a user name for
an account that has no password is able to connect
successfully to the server.
MySQL user names can be up to 16 characters long. Operating
system user names, because they are completely unrelated to
MySQL user names, may be of a different maximum length. For
example, Unix user names typically are limited to eight
characters.
Warning
The limit on MySQL user name length is hard-coded in the
MySQL servers and clients, and trying to circumvent it by
modifying the definitions of the tables in the
mysql database does not
work.
You should never alter any of the tables in the
mysql database in any manner whatsoever
except by means of the procedure that is described in
Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”. Attempting to redefine
MySQL's system tables in any other fashion results in
undefined (and unsupported!) behavior.
MySQL passwords have nothing to do with passwords for logging
in to your operating system. There is no necessary connection
between the password you use to log in to a Windows or Unix
machine and the password you use to access the MySQL server on
that machine.
MySQL encrypts passwords using its own algorithm. This
encryption is the same as that implemented by the
PASSWORD() SQL function but
differs from that used during the Unix login process. Unix
password encryption is the same as that implemented by the
ENCRYPT() SQL function. See the
descriptions of the PASSWORD()
and ENCRYPT() functions in
Section 11.10.2, “Encryption and Compression Functions”.
From version 4.1 on, MySQL employs a stronger authentication
method that has better password protection during the
connection process than in earlier versions. It is secure even
if TCP/IP packets are sniffed or the mysql
database is captured. (In earlier versions, even though
passwords are stored in encrypted form in the
user table, knowledge of the encrypted
password value could be used to connect to the MySQL server.)
Section 5.5.6.3, “Password Hashing in MySQL”, discusses password
encryption further.
When you install MySQL, the grant tables are populated with an
initial set of accounts. These accounts have names and access
privileges that are described in
Section 2.17.3, “Securing the Initial MySQL Accounts”, which also discusses how to
assign passwords to them. Thereafter, you normally set up, modify,
and remove MySQL accounts using statements such as
GRANT and
REVOKE. See
Section 12.5.1, “Account Management Statements”.
When you connect to a MySQL server with a command-line client, you
should specify the user name and password for the account that you
want to use:
shell> mysql --user=monty --password=guess db_name
If you prefer short options, the command looks like this:
shell> mysql -u monty -pguess db_name
There must be no space between the
-p option and the following password value. For
additional information about specifying user names, passwords, and
other connection parameters, see Section 4.2.2, “Connecting to the MySQL Server”.
5.5.2. Adding User Accounts
You can create MySQL accounts in two ways:
By using statements intended for creating accounts, such as
CREATE USER or
GRANT. These statements cause
the server to make appropriate modifications to the grant
tables.
By manipulating the MySQL grant tables directly with
statements such as INSERT,
UPDATE, or
DELETE.
The preferred method is to use account-creation statements because
they are more concise and less error-prone than manipulating the
grant tables directly. CREATE USER
and GRANT are described in
Section 12.5.1, “Account Management Statements”.
Another option for creating accounts is to use one of several
available third-party programs that offer capabilities for MySQL
account administration. phpMyAdmin is one such
program.
The following examples show how to use the
mysql client program to set up new accounts.
These examples assume that privileges have been set up according
to the defaults described in Section 2.17.3, “Securing the Initial MySQL Accounts”.
This means that to make changes, you must connect to the MySQL
server as the MySQL root user, and the
root account must have the
INSERT privilege for the
mysql database and the
RELOAD administrative privilege.
As noted in the examples where appropriate, some of the statements
will fail if the server's SQL mode has been set to enable certain
restrictions. In particular, strict mode
(STRICT_TRANS_TABLES,
STRICT_ALL_TABLES) and
NO_AUTO_CREATE_USER will prevent
the server from accepting some of the statements. Workarounds are
indicated for these cases. For more information about SQL modes
and their effect on grant table manipulation, see
Section 5.1.7, “Server SQL Modes”, and Section 12.5.1.3, “GRANT Syntax”.
First, use the mysql program to connect to the
server as the MySQL root user:
shell> mysql --user=root mysql
If you have assigned a password to the root
account, you'll also need to supply a --password
or -p option, both for this
mysql command and for those later in this
section.
After connecting to the server as root, you can
add new accounts. The following statements use
GRANT to set up four new accounts:
mysql> CREATE USER 'monty'@'localhost' IDENTIFIED BY 'some_pass';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'
-> WITH GRANT OPTION;
mysql> CREATE USER 'monty'@'%' IDENTIFIED BY 'some_pass';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'
-> WITH GRANT OPTION;
mysql> CREATE USER 'admin'@'localhost';
mysql> GRANT RELOAD,PROCESS ON *.* TO 'admin'@'localhost';
mysql> CREATE USER 'dummy'@'localhost';
The accounts created by these statements have the following
properties:
Two of the accounts have a user name of
monty and a password of
some_pass. Both accounts are superuser
accounts with full privileges to do anything. The
'monty'@'localhost' account can be used
only when connecting from the local host. The
'monty'@'%' account uses the
'%' wildcard for the host part, so it can
be used to connect from any host.
It is necessary to have both accounts for
monty to be able to connect from anywhere
as monty. Without the
localhost account, the anonymous-user
account for localhost that is created by
mysql_install_db would take precedence when
monty connects from the local host. As a
result, monty would be treated as an
anonymous user. The reason for this is that the anonymous-user
account has a more specific Host column
value than the 'monty'@'%' account and thus
comes earlier in the user table sort order.
(user table sorting is discussed in
Section 5.4.4, “Access Control, Stage 1: Connection Verification”.)
The 'admin'@'localhost' account has no
password. This account can be used only by
admin to connect from the local host. It is
granted the RELOAD and
PROCESS administrative
privileges. These privileges allow the
admin user to execute the
mysqladmin reload, mysqladmin
refresh, and mysqladmin
flush-xxx commands, as
well as mysqladmin processlist . No
privileges are granted for accessing any databases. You could
add such privileges later by issuing additional
GRANT statements.
The 'dummy'@'localhost' account has no
password. This account can be used only to connect from the
local host. No privileges are granted. It is assumed that you
will grant specific privileges to the account later.
The statements that create accounts with no password will fail if
the NO_AUTO_CREATE_USER SQL mode
is enabled. To deal with this, use an IDENTIFIED
BY clause that specifies a nonempty password.
To check the privileges for an account, use
SHOW GRANTS:
mysql> SHOW GRANTS FOR 'admin'@'localhost';
+-----------------------------------------------------+
| Grants for admin@localhost |
+-----------------------------------------------------+
| GRANT RELOAD, PROCESS ON *.* TO 'admin'@'localhost' |
+-----------------------------------------------------+
As an alternative to CREATE USER
and GRANT, you can create the same
accounts directly by issuing INSERT
statements and then telling the server to reload the grant tables
using FLUSH
PRIVILEGES:
shell> mysql --user=root mysql
mysql> INSERT INTO user
-> VALUES('localhost','monty',PASSWORD('some_pass'),
-> 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user
-> VALUES('%','monty',PASSWORD('some_pass'),
-> 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y',
-> 'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y',
-> '','','','',0,0,0,0);
mysql> INSERT INTO user SET Host='localhost',User='admin',
-> Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;
When you create accounts with
INSERT, it is necessary to use
FLUSH PRIVILEGES
to tell the server to reload the grant tables. Otherwise, the
changes go unnoticed until you restart the server. With
CREATE USER,
FLUSH PRIVILEGES
is unnecessary.
The reason for using the PASSWORD()
function with INSERT is to encrypt
the password. The CREATE USER
statement encrypts the password for you, so
PASSWORD() is unnecessary.
The 'Y' values enable privileges for the
accounts. Depending on your MySQL version, you may have to use a
different number of 'Y' values in the first two
INSERT statements. The
INSERT statement for the
admin account employs the more readable
extended INSERT syntax using
SET.
In the INSERT statement for the
dummy account, only the
Host, User, and
Password columns in the user
table row are assigned values. None of the privilege columns are
set explicitly, so MySQL assigns them all the default value of
'N'. This is equivalent to what
CREATE USER does.
If strict SQL mode is enabled, all columns that have no default
value must have a value specified. In this case,
INSERT statements must explicitly
specify values for the
ssl_cipher,
x509_issuer, and
x509_subject columns.
To set up a superuser account, it is necessary only to create a
user table entry with the privilege columns set
to 'Y'. The user table
privileges are global, so no entries in any of the other grant
tables are needed.
The next examples create three accounts and give them access to
specific databases. Each of them has a user name of
custom and password of
obscure.
To create the accounts with CREATE
USER and GRANT, use the
following statements:
shell> mysql --user=root mysql
mysql> CREATE USER 'custom'@'localhost' IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
-> ON bankaccount.*
-> TO 'custom'@'localhost';
mysql> CREATE USER 'custom'@'host47.example.com' IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
-> ON expenses.*
-> TO 'custom'@'host47.example.com';
mysql> CREATE USER 'custom'@'server.domain' IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
-> ON customer.*
-> TO 'custom'@'server.domain';
The three accounts can be used as follows:
The first account can access the
bankaccount database, but only from the
local host.
The second account can access the expenses
database, but only from the host
host47.example.com.
The third account can access the customer
database, but only from the host
server.domain.
To set up the custom accounts without
GRANT, use
INSERT statements as follows to
modify the grant tables directly:
shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('localhost','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('host47.example.com','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('server.domain','custom',PASSWORD('obscure'));
mysql> INSERT INTO db
-> (Host,Db,User,Select_priv,Insert_priv,
-> Update_priv,Delete_priv,Create_priv,Drop_priv)
-> VALUES('localhost','bankaccount','custom',
-> 'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
-> (Host,Db,User,Select_priv,Insert_priv,
-> Update_priv,Delete_priv,Create_priv,Drop_priv)
-> VALUES('host47.example.com','expenses','custom',
-> 'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
-> (Host,Db,User,Select_priv,Insert_priv,
-> Update_priv,Delete_priv,Create_priv,Drop_priv)
-> VALUES('server.domain','customer','custom',
-> 'Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;
The first three INSERT statements
add user table entries that allow the user
custom to connect from the various hosts with
the given password, but grant no global privileges (all privileges
are set to the default value of 'N'). The next
three INSERT statements add
db table entries that grant privileges to
custom for the bankaccount,
expenses, and customer
databases, but only when accessed from the proper hosts. As usual
when you modify the grant tables directly, you must tell the
server to reload them with
FLUSH PRIVILEGES
so that the privilege changes take effect.
To create a user who has access from all machines in a given
domain (for example, mydomain.com), you can use
the “%” wildcard character in the
host part of the account name:
mysql> CREATE USER 'myname'@'%.mydomain.com' IDENTIFIED BY 'mypass';
To do the same thing by modifying the grant tables directly, do
this:
mysql> INSERT INTO user (Host,User,Password,...)
-> VALUES('%.mydomain.com','myname',PASSWORD('mypass'),...);
mysql> FLUSH PRIVILEGES;
5.5.4. Limiting Account Resources
One means of limiting use of MySQL server resources is to set the
max_user_connections system
variable to a nonzero value. However, this limits only the number
of simultaneous connections made using a single account, and not
what a client can do once connected. In addition, this method is
strictly global, and does not allow for management of individual
accounts. Both types of control are of interest to many MySQL
administrators, particularly those working for Internet Service
Providers.
In MySQL 5.0, you can limit the following server
resources for individual accounts:
The number of queries that an account can issue per hour
The number of updates that an account can issue per hour
The number of times an account can connect to the server per
hour
The number of simultaneous connections to the server an
account can have (as of MySQL 5.0.3)
Any statement that a client can issue counts against the query
limit. Only statements that modify databases or tables count
against the update limit.
Before MySQL 5.0.3, an “account” in this context is
assessed against the actual host from which a user connects.
Suppose that there is a row in the user table
that has User and Host
values of usera and
%.example.com, to allow
usera to connect from any host in the
example.com domain. If usera
connects simultaneously from host1.example.com
and host2.example.com, the server applies the
account resource limits separately to each connection. If
usera connects again from
host1.example.com, the server applies the
limits for that connection together with the existing connection
from that host.
As of MySQL 5.0.3, an “account” corresponds to a
single row in the user table. That is,
connections are assessed against the User and
Host value in the user table
row that applies to the connection. In this case, the server
applies resource limits collectively to all connections by
usera from any host in the
example.com domain because all such connections
use the same account. The pre-5.0.3 method of accounting may be
selected by starting the server with the
--old-style-user-limits option.
The server limits account resources based on the resource-related
columns of the user table in the
mysql database:
max_questions, max_updates,
max_connections, and
max_user_connections. If your
user table does not have these columns, it must
be upgraded; see Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
To set resource limits, use the
GRANT statement and provide a
WITH clause that names each resource to be
limited. For example, to create a new account that can access the
customer database, but only in a limited
fashion, issue these statements:
mysql> CREATE USER 'francis'@'localhost' IDENTIFIED BY 'frank';
mysql> GRANT ALL ON customer.* TO 'francis'@'localhost'
-> WITH MAX_QUERIES_PER_HOUR 20
-> MAX_UPDATES_PER_HOUR 10
-> MAX_CONNECTIONS_PER_HOUR 5
-> MAX_USER_CONNECTIONS 2;
The limit types need not all be named in the
WITH clause, but those named can be present in
any order. The value for each per-hour limit should be an integer
representing a count per hour. If the
GRANT statement has no
WITH clause, the limits are each set to the
default value of zero (that is, no limit). For
MAX_USER_CONNECTIONS, the limit is an integer
representing the maximum number of simultaneous connections the
account can make at any one time. If the limit is set to the
default value of zero, the
max_user_connections system
variable determines the number of simultaneous connections for the
account.
To modify limits for an existing account, use a
GRANT USAGE
statement at the global level (ON *.*). The
following statement changes the query limit for
francis to 100:
mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
-> WITH MAX_QUERIES_PER_HOUR 100;
This statement leaves the account's existing privileges unchanged
and modifies only the limit values specified.
To remove an existing limit, set its value to zero. For example,
to remove the limit on how many times per hour
francis can connect, use this statement:
mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
-> WITH MAX_CONNECTIONS_PER_HOUR 0;
Resource-use counting takes place when any account has a nonzero
limit placed on its use of any of the resources.
As the server runs, it counts the number of times each account
uses resources. If an account reaches its limit on number of
connections within the last hour, further connections for the
account are rejected until that hour is up. Similarly, if the
account reaches its limit on the number of queries or updates,
further queries or updates are rejected until the hour is up. In
all such cases, an appropriate error message is issued.
Resource counting is done per account, not per client. For
example, if your account has a query limit of 50, you cannot
increase your limit to 100 by making two simultaneous client
connections to the server. Queries issued on both connections are
counted together.
Queries for which results are served from the query cache do not
count against the MAX_QUERIES_PER_HOUR limit.
The current per-hour resource-use counts can be reset globally for
all accounts, or individually for a given account:
To reset the current counts to zero for all accounts, issue a
FLUSH
USER_RESOURCES statement. The counts also can be
reset by reloading the grant tables (for example, with a
FLUSH
PRIVILEGES statement or a mysqladmin
reload command).
The counts for an individual account can be set to zero by
re-granting it any of its limits. To do this, use
GRANT USAGE
as described earlier and specify a limit value equal to the
value that the account currently has.
Counter resets do not affect the
MAX_USER_CONNECTIONS limit.
All counts begin at zero when the server starts; counts are not
carried over through a restart.
For the MAX_USER_CONNECTIONS limit, an edge
case can occur if the account currently has open the maximum
number of connections allowed to it: A disconnect followed quickly
by a connect can result in an error
(ER_TOO_MANY_USER_CONNECTIONS or
ER_USER_LIMIT_REACHED) if the
server has not fully processed the disconnect by the time the
connect occurs. When the server finishes disconnect processing,
another connection will once more be allowed.
5.5.5. Assigning Account Passwords
To assign a password when you create a new account with
CREATE USER, include an
IDENTIFIED BY clause:
mysql> CREATE USER 'jeffrey'@'localhost' IDENTIFIED BY 'biscuit';
To assign or change a password for an existing account, one way is
to issue a SET PASSWORD statement:
mysql> SET PASSWORD FOR 'jeffrey'@'localhost' = PASSWORD('biscuit');
Only users such as root that have update access
to the mysql database can change the password
for other users. If you are not connected as an anonymous user,
you can change your own password by omitting the
FOR clause:
mysql> SET PASSWORD = PASSWORD('biscuit');
You can also use a GRANT
USAGE statement at the global level (ON
*.*) to assign a password to an account without
affecting the account's current privileges:
mysql> GRANT USAGE ON *.* TO 'jeffrey'@'localhost' IDENTIFIED BY 'biscuit';
Passwords can be assigned from the command line by using the
mysqladmin command:
shell> mysqladmin -u user_name -h host_name password "newpwd"
The account for which this command resets the password is the one
with a user table row that matches
user_name in the
User column and the client host from
which you connect in the Host
column.
Although it is generally preferable to assign passwords using one
of the preceding methods, you can also do so by modifying the
user table directly:
To establish a password when creating a new account, provide a
value for the Password column:
shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('localhost','jeffrey',PASSWORD('biscuit'));
mysql> FLUSH PRIVILEGES;
To change the password for an existing account, use
UPDATE to set the
Password column value:
shell> mysql -u root mysql
mysql> UPDATE user SET Password = PASSWORD('bagel')
-> WHERE Host = 'localhost' AND User = 'francis';
mysql> FLUSH PRIVILEGES;
When you assign passwords using CREATE
USER or GRANT with an
IDENTIFIED BY clause or with the
mysqladmin password command, they take care of
encrypting the password for you.
When you assign an account a nonempty password using
SET PASSWORD,
INSERT, or
UPDATE, you must use the
PASSWORD() function to encrypt the
password. PASSWORD() is necessary
because the user table stores passwords in
encrypted form, not as plaintext. If you forget that fact, you are
likely to set passwords like this:
shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
-> VALUES('localhost','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;
The result is that the literal value 'biscuit'
is stored as the password in the user table,
not the encrypted value. When jeffrey attempts
to connect to the server using this password, the value is
encrypted and compared to the value stored in the
user table. However, the stored value is the
literal string 'biscuit', so the comparison
fails and the server rejects the connection:
shell> mysql -u jeffrey -pbiscuit test
Access denied
5.5.6. Password Security in MySQL
Passwords occur in several contexts within MySQL. The following
sections provide guidelines that enable administrators and end
users to keep these passwords secure and avoid exposing them.
There is also a discussion of how MySQL uses password hashing
internally.
5.5.6.1. Administrator Guidelines for Password Security
Database administrators should use the following guidelines to
keep passwords secure.
MySQL stores passwords for user accounts in the
mysql.user table. Access to this table should
never be granted to any nonadministrative accounts.
Passwords can appear as plain text in SQL statements such as
CREATE USER,
GRANT, and
SET PASSWORD. If these statements
are logged by the MySQL server, the passwords become available
to anyone with access to the logs. This applies to the general
query log, the slow query log, and the binary log (see
Section 5.2, “MySQL Server Logs”). To guard against unwarranted
exposure to log files, they should be located in a directory
that restricts access to only the server and the database
administrator.
Replication slaves store the password for the replication master
in the master.info file. Access to this
file should be restricted to the database adminstrator.
Database backups that include tables or log files containing
passwords should be protected using a restricted access mode.
5.5.6.2. End-User Guidelines for Password Security
MySQL users should use the following guidelines to keep
passwords secure.
When you run a client program to connect to the MySQL server, it
is inadvisable to specify your password in a way that exposes it
to discovery by other users. The methods you can use to specify
your password when you run client programs are listed here,
along with an assessment of the risks of each method. In short,
the safest methods are to have the client program prompt for the
password or to specify the password in a properly protected
option file.
Use a
-pyour_pass or
--password=your_pass
option on the command line. For example:
shell> mysql -u francis -pfrank db_name
This is convenient but insecure,
because your password becomes visible to system status
programs such as ps that may be invoked
by other users to display command lines. MySQL clients
typically overwrite the command-line password argument with
zeros during their initialization sequence. However, there
is still a brief interval during which the value is visible.
Also, on some systems this overwriting strategy is
ineffective and the password remains visible to
ps. (SystemV Unix systems and perhaps
others are subject to this problem.)
If your operating environment is set up to display your
current command in the title bar of your terminal window,
the password remains visible as long as the command is
running, even if the command has scrolled out of view in the
window content area.
Use the -p or --password
option on the command line with no password value specified.
In this case, the client program solicits the password
interactively:
shell> mysql -u francis -p db_name
Enter password: ********
The “*” characters indicate
where you enter your password. The password is not displayed
as you enter it.
It is more secure to enter your password this way than to
specify it on the command line because it is not visible to
other users. However, this method of entering a password is
suitable only for programs that you run interactively. If
you want to invoke a client from a script that runs
noninteractively, there is no opportunity to enter the
password from the keyboard. On some systems, you may even
find that the first line of your script is read and
interpreted (incorrectly) as your password.
Store your password in an option file. For example, on Unix
you can list your password in the
[client] section of the
.my.cnf file in your home directory:
[client]
password=your_pass
To keep the password safe, the file should not be accessible
to anyone but yourself. To ensure this, set the file access
mode to 400 or 600.
For example:
shell> chmod 600 .my.cnf
Section 4.2.3.3, “Using Option Files”, discusses option files in
more detail.
Store your password in the MYSQL_PWD
environment variable. See
Section 2.20, “Environment Variables”.
This method of specifying your MySQL password must be
considered extremely insecure and
should not be used. Some versions of ps
include an option to display the environment of running
processes. If you set MYSQL_PWD, your
password is exposed to any other user who runs
ps. Even on systems without such a
version of ps, it is unwise to assume
that there are no other methods by which users can examine
process environments.
On Unix, the mysql client writes a record of
executed statements to a history file (see
Section 4.5.1, “mysql — The MySQL Command-Line Tool”). By default, this file is named
.mysql_history and is created in your home
directory. Passwords can appear as plain text in SQL statements
such as CREATE USER,
GRANT, and
SET PASSWORD, so if you use these
statements, they are logged in the history file. To keep this
file safe, use a restrictive access mode, the same way as
described earlier for the .my.cnf file.
If your command interpreter is configured to maintain a history,
any file in which the commands are saved will contain MySQL
passwords entered on the command line. For example,
bash uses
~/.bash_history. Any such file should had a
restrictive access mode.
5.5.6.3. Password Hashing in MySQL
MySQL user accounts are listed in the user
table of the mysql database. Each MySQL
account is assigned a password, although what is stored in the
Password column of the
user table is not the plaintext version of
the password, but a hash value computed from it. Password hash
values are computed by the
PASSWORD() function.
MySQL uses passwords in two phases of client/server
communication:
When a client attempts to connect to the server, there is an
initial authentication step in which the client must present
a password that has a hash value matching the hash value
stored in the user table for the account
that the client wants to use.
After the client connects, it can (if it has sufficient
privileges) set or change the password hashes for accounts
listed in the user table. The client can
do this by using the
PASSWORD() function to
generate a password hash, or by using the
GRANT or
SET PASSWORD statements.
In other words, the server uses hash values
during authentication when a client first attempts to connect.
The server generates hash values if a
connected client invokes the
PASSWORD() function or uses a
GRANT or SET
PASSWORD statement to set or change a password.
The password hashing mechanism was updated in MySQL 4.1 to
provide better security and to reduce the risk of passwords
being intercepted. However, this new mechanism is understood
only by MySQL 4.1 (and newer) servers and clients, which can
result in some compatibility problems. A 4.1 or newer client can
connect to a pre-4.1 server, because the client understands both
the old and new password hashing mechanisms. However, a pre-4.1
client that attempts to connect to a 4.1 or newer server may run
into difficulties. For example, a 3.23 mysql
client that attempts to connect to a 5.0 server may
fail with the following error message:
shell> mysql -h localhost -u root
Client does not support authentication protocol requested
by server; consider upgrading MySQL client
Another common example of this phenomenon occurs for attempts to
use the older PHP mysql extension after
upgrading to MySQL 4.1 or newer. (See
Section 20.9.6, “Common Problems with MySQL and PHP”.)
The following discussion describes the differences between the
old and new password mechanisms, and what you should do if you
upgrade your server but need to maintain backward compatibility
with pre-4.1 clients. Additional information can be found in
Section B.5.2.4, “Client does not support authentication protocol”. This information is of particular
importance to PHP programmers migrating MySQL databases from
version 4.0 or lower to version 4.1 or higher.
Note
This discussion contrasts 4.1 behavior with pre-4.1 behavior,
but the 4.1 behavior described here actually begins with
4.1.1. MySQL 4.1.0 is an “odd” release because it
has a slightly different mechanism than that implemented in
4.1.1 and up. Differences between 4.1.0 and more recent
versions are described further in MySQL 3.23, 4.0, 4.1 Reference Manual.
Prior to MySQL 4.1, password hashes computed by the
PASSWORD() function are 16 bytes
long. Such hashes look like this:
mysql> SELECT PASSWORD('mypass');
+--------------------+
| PASSWORD('mypass') |
+--------------------+
| 6f8c114b58f2ce9e |
+--------------------+
The Password column of the
user table (in which these hashes are stored)
also is 16 bytes long before MySQL 4.1.
As of MySQL 4.1, the PASSWORD()
function has been modified to produce a longer 41-byte hash
value:
mysql> SELECT PASSWORD('mypass');
+-------------------------------------------+
| PASSWORD('mypass') |
+-------------------------------------------+
| *6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4 |
+-------------------------------------------+
Accordingly, the Password column in the
user table also must be 41 bytes long to
store these values:
If you perform a new installation of MySQL 5.0,
the Password column is made 41 bytes long
automatically.
Upgrading from MySQL 4.1 (4.1.1 or later in the 4.1 series)
to MySQL 5.0 should not give rise to any issues
in this regard because both versions use the same password
hashing mechanism. If you wish to upgrade an older release
of MySQL to version 5.0, you should upgrade to
version 4.1 first, then upgrade the 4.1 installation to
5.0.
A widened Password column can store password
hashes in both the old and new formats. The format of any given
password hash value can be determined two ways:
The obvious difference is the length (16 bytes versus 41
bytes).
A second difference is that password hashes in the new
format always begin with a
“*” character, whereas
passwords in the old format never do.
The longer password hash format has better cryptographic
properties, and client authentication based on long hashes is
more secure than that based on the older short hashes.
The differences between short and long password hashes are
relevant both for how the server uses passwords during
authentication and for how it generates password hashes for
connected clients that perform password-changing operations.
The way in which the server uses password hashes during
authentication is affected by the width of the
Password column:
If the column is short, only short-hash authentication is
used.
If the column is long, it can hold either short or long
hashes, and the server can use either format:
Pre-4.1 clients can connect, although because they know
only about the old hashing mechanism, they can
authenticate only using accounts that have short hashes.
4.1 and later clients can authenticate using accounts
that have short or long hashes.
Even for short-hash accounts, the authentication process is
actually a bit more secure for 4.1 and later clients than for
older clients. In terms of security, the gradient from least to
most secure is:
Pre-4.1 client authenticating with short password hash
4.1 or later client authenticating with short password hash
4.1 or later client authenticating with long password hash
The way in which the server generates password hashes for
connected clients is affected by the width of the
Password column and by the
--old-passwords option. A 4.1 or
later server generates long hashes only if certain conditions
are met: The Password column must be wide
enough to hold long values and the
--old-passwords option must not
be given. These conditions apply as follows:
The Password column must be wide enough
to hold long hashes (41 bytes). If the column has not been
updated and still has the pre-4.1 width of 16 bytes, the
server notices that long hashes cannot fit into it and
generates only short hashes when a client performs
password-changing operations using
PASSWORD(),
GRANT, or
SET PASSWORD. This is the
behavior that occurs if you have upgraded to 4.1 but have
not yet run the mysql_upgrade program to
widen the Password column.
If the Password column is wide, it can
store either short or long password hashes. In this case,
PASSWORD(),
GRANT, and
SET PASSWORD generate long
hashes unless the server was started with the
--old-passwords option. That
option forces the server to generate short password hashes
instead.
The purpose of the
--old-passwords option is to
enable you to maintain backward compatibility with pre-4.1
clients under circumstances where the server would otherwise
generate long password hashes. The option doesn't affect
authentication (4.1 and later clients can still use accounts
that have long password hashes), but it does prevent creation of
a long password hash in the user table as the
result of a password-changing operation. Were that to occur, the
account no longer could be used by pre-4.1 clients. Without the
--old-passwords option, the
following undesirable scenario is possible:
An old client connects to an account that has a short
password hash.
The client changes its own password. Without
--old-passwords, this results
in the account having a long password hash.
The next time the old client attempts to connect to the
account, it cannot, because the account has a long password
hash that requires the new hashing mechanism during
authentication. (Once an account has a long password hash in
the user table, only 4.1 and later clients can authenticate
for it, because pre-4.1 clients do not understand long
hashes.)
This scenario illustrates that, if you must support older
pre-4.1 clients, it is dangerous to run a 4.1 or newer server
without using the --old-passwords
option. By running the server with
--old-passwords,
password-changing operations do not generate long password
hashes and thus do not cause accounts to become inaccessible to
older clients. (Those clients cannot inadvertently lock
themselves out by changing their password and ending up with a
long password hash.)
The downside of the
--old-passwords option is that
any passwords you create or change use short hashes, even for
4.1 clients. Thus, you lose the additional security provided by
long password hashes. If you want to create an account that has
a long hash (for example, for use by 4.1 clients), you must do
so while running the server without
--old-passwords.
MySQL Enterprise
Subscribers to the MySQL Enterprise Monitor are automatically
alerted whenever a server is running with the
--old-passwords option. For
more information, see
http://www.mysql.com/products/enterprise/advisors.html.
The following scenarios are possible for running a 4.1 or later
server:
Scenario 1: Short
Password column in user table:
Only short hashes can be stored in the
Password column.
The server uses only short hashes during client
authentication.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use short hashes
exclusively. Any change to an account's password results in
that account having a short password hash.
The --old-passwords option
can be used but is superfluous because with a short
Password column, the server generates
only short password hashes anyway.
Scenario 2: Long
Password column; server not started with
--old-passwords option:
Short or long hashes can be stored in the
Password column.
4.1 and later clients can authenticate using accounts that
have short or long hashes.
Pre-4.1 clients can authenticate only using accounts that
have short hashes.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use long hashes
exclusively. A change to an account's password results in
that account having a long password hash.
As indicated earlier, a danger in this scenario is that it is
possible for accounts that have a short password hash to become
inaccessible to pre-4.1 clients. A change to such an account's
password made via GRANT,
PASSWORD(), or
SET PASSWORD results in the
account being given a long password hash. From that point on, no
pre-4.1 client can authenticate to that account until the client
upgrades to 4.1.
To deal with this problem, you can change a password in a
special way. For example, normally you use
SET PASSWORD as follows to change
an account password:
SET PASSWORD FOR 'some_user'@'some_host' = PASSWORD('mypass');
To change the password but create a short hash, use the
OLD_PASSWORD() function instead:
SET PASSWORD FOR 'some_user'@'some_host' = OLD_PASSWORD('mypass');
OLD_PASSWORD() is useful for
situations in which you explicitly want to generate a short
hash.
Scenario 3: Long
Password column; 4.1 or newer server started
with --old-passwords option:
Short or long hashes can be stored in the
Password column.
4.1 and later clients can authenticate for accounts that
have short or long hashes (but note that it is possible to
create long hashes only when the server is started without
--old-passwords).
Pre-4.1 clients can authenticate only for accounts that have
short hashes.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use short hashes
exclusively. Any change to an account's password results in
that account having a short password hash.
In this scenario, you cannot create accounts that have long
password hashes, because the
--old-passwords option prevents
generation of long hashes. Also, if you create an account with a
long hash before using the
--old-passwords option, changing
the account's password while
--old-passwords is in effect
results in the account being given a short password, causing it
to lose the security benefits of a longer hash.
The disadvantages for these scenarios may be summarized as
follows:
In scenario 1, you cannot take advantage of longer hashes that
provide more secure authentication.
In scenario 2, accounts with short hashes become inaccessible to
pre-4.1 clients if you change their passwords without explicitly
using OLD_PASSWORD().
In scenario 3, --old-passwords
prevents accounts with short hashes from becoming inaccessible,
but password-changing operations cause accounts with long hashes
to revert to short hashes, and you cannot change them back to
long hashes while --old-passwords
is in effect.
5.5.6.4. Implications of Password Hashing Changes in MySQL 4.1 for Application
Programs
An upgrade to MySQL version 4.1 or later can cause compatibility
issues for applications that use
PASSWORD() to generate passwords
for their own purposes. Applications really should not do this,
because PASSWORD() should be used
only to manage passwords for MySQL accounts. But some
applications use PASSWORD() for
their own purposes anyway.
If you upgrade to 4.1 or later from a pre-4.1 version of MySQL
and run the server under conditions where it generates long
password hashes, an application using
PASSWORD() for its own passwords
breaks. The recommended course of action in such cases is to
modify the application to use another function, such as
SHA1() or
MD5(), to produce hashed values.
If that is not possible, you can use the
OLD_PASSWORD() function, which is
provided for generate short hashes in the old format. However,
you should note that
OLD_PASSWORD() may one day no
longer be supported.
If the server is running under circumstances where it generates
short hashes, OLD_PASSWORD() is
available but is equivalent to
PASSWORD().
PHP programmers migrating their MySQL databases from version 4.0
or lower to version 4.1 or higher should see
Section 20.9, “MySQL PHP API”.
5.5.7. Using SSL for Secure Connections
MySQL supports secure (encrypted) connections between MySQL
clients and the server using the Secure Sockets Layer (SSL)
protocol. This section discusses how to use SSL connections. For
information on how to require users to use SSL connections, see
the discussion of the REQUIRE clause of the
GRANT statement in
Section 12.5.1.3, “GRANT Syntax”.
The standard configuration of MySQL is intended to be as fast as
possible, so encrypted connections are not used by default. Doing
so would make the client/server protocol much slower. Encrypting
data is a CPU-intensive operation that requires the computer to do
additional work and can delay other MySQL tasks. For applications
that require the security provided by encrypted connections, the
extra computation is warranted.
MySQL allows encryption to be enabled on a per-connection basis.
You can choose a normal unencrypted connection or a secure
encrypted SSL connection according the requirements of individual
applications.
Secure connections are based on the OpenSSL API and are available
through the MySQL C API. Replication uses the C API, so secure
connections can be used between master and slave servers.
Another way to connect securely is from within an SSH connection
to the MySQL server host. For an example, see
Section 5.5.8, “Connecting to MySQL Remotely from Windows with SSH”.
5.5.7.1. Basic SSL Concepts
To understand how MySQL uses SSL, it is necessary to explain
some basic SSL and X509 concepts. People who are familiar with
these can skip this part of the discussion.
By default, MySQL uses unencrypted connections between the
client and the server. This means that someone with access to
the network could watch all your traffic and look at the data
being sent or received. They could even change the data while it
is in transit between client and server. To improve security a
little, you can compress client/server traffic by using the
--compress option when invoking client
programs. However, this does not foil a determined attacker.
When you need to move information over a network in a secure
fashion, an unencrypted connection is unacceptable. Encryption
is the way to make any kind of data unreadable. In fact, today's
practice requires many additional security elements from
encryption algorithms. They should resist many kind of known
attacks such as changing the order of encrypted messages or
replaying data twice.
SSL is a protocol that uses different encryption algorithms to
ensure that data received over a public network can be trusted.
It has mechanisms to detect any data change, loss, or replay.
SSL also incorporates algorithms that provide identity
verification using the X509 standard.
X509 makes it possible to identify someone on the Internet. It
is most commonly used in e-commerce applications. In basic
terms, there should be some company called a “Certificate
Authority” (or CA) that assigns electronic certificates
to anyone who needs them. Certificates rely on asymmetric
encryption algorithms that have two encryption keys (a public
key and a secret key). A certificate owner can show the
certificate to another party as proof of identity. A certificate
consists of its owner's public key. Any data encrypted with this
public key can be decrypted only using the corresponding secret
key, which is held by the owner of the certificate.
If you need more information about SSL, X509, or encryption, use
your favorite Internet search engine to search for the keywords
in which you are interested.
5.5.7.2. Using SSL Connections
To use SSL connections between the MySQL server and client
programs, your system must support either OpenSSL or yaSSL and
your version of MySQL must be built with SSL support.
To make it easier to use secure connections, MySQL is bundled
with yaSSL as of MySQL 5.0.10. (MySQL and yaSSL employ the same
licensing model, whereas OpenSSL uses an Apache-style license.)
yaSSL support initially was available only for a few platforms,
but now it is available on all platforms supported by Sun
Microsystems, Inc.
To get secure connections to work with MySQL and SSL, you must
do the following:
If you are not using a binary (precompiled) version of MySQL
that has been built with SSL support, and you are going to
use OpenSSL rather than the bundled yaSSL library, install
OpenSSL if it has not already been installed. We have tested
MySQL with OpenSSL 0.9.6. To obtain OpenSSL, visit
http://www.openssl.org.
Building MySQL using OpenSSL requires a shared OpenSSL
library, otherwise linker errors occur. Alternatively, build
MySQL using yaSSL.
If you are not using a binary (precompiled) version of MySQL
that has been built with SSL support, configure a MySQL
source distribution to use SSL. When you configure MySQL,
invoke the configure script with the
appropriate option to select the SSL library that you want
to use.
For yaSSL:
shell> ./configure --with-yassl
For OpenSSL:
shell> ./configure --with-openssl
Before MySQL 5.0, it was also neccessary to use
--with-vio, but that option is no longer
required.
Note that yaSSL support on Unix platforms requires that
either /dev/urandom or
/dev/random be available to retrieve
true random numbers. For additional information (especially
regarding yaSSL on Solaris versions prior to 2.8 and HP-UX),
see Bug#13164.
Make sure that the user in the
mysql database includes the SSL-related
columns (beginning with ssl_ and
x509_). If your user
table does not have these columns, it must be upgraded; see
Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”.
To check whether a server binary is compiled with SSL
support, invoke it with the
--ssl option. An error will
occur if the server does not support SSL:
shell> mysqld --ssl --help
060525 14:18:52 [ERROR] mysqld: unknown option '--ssl'
To check whether a running mysqld server
supports SSL, examine the value of the
have_ssl system variable
(if you have no have_ssl
variable, check for
have_openssl):
mysql> SHOW VARIABLES LIKE 'have_ssl';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_ssl | YES |
+---------------+-------+
If the value is YES, the server supports
SSL connections. If the value is
DISABLED, the server supports SSL
connections but was not started with the appropriate
--ssl-xxx
options (described later in this section).
To enable SSL connections, the proper SSL-related options must
be used (see Section 5.5.7.3, “SSL Command Options”).
To start the MySQL server so that it allows clients to connect
via SSL, use the options that identify the key and certificate
files the server needs when establishing a secure connection:
shell> mysqld --ssl-ca=cacert.pem \
--ssl-cert=server-cert.pem \
--ssl-key=server-key.pem
--ssl-ca identifies the
Certificate Authority (CA) certificate.
--ssl-cert identifies the
server public key. This can be sent to the client and
authenticated against the CA certificate that it has.
--ssl-key identifies the
server private key.
To establish a secure connection to a MySQL server with SSL
support, the options that a client must specify depend on the
SSL requirements of the user account that the client uses. (See
the discussion of the REQUIRE clause in
Section 12.5.1.3, “GRANT Syntax”.)
If the account has no special SSL requirements or was created
using a GRANT statement that
includes the REQUIRE SSL option, a client can
connect securely by using just the
--ssl-ca option:
shell> mysql --ssl-ca=cacert.pem
To require that a client certificate also be specified, create
the account using the REQUIRE X509 option.
Then the client must also specify the proper client key and
certificate files or the server will reject the connection:
shell> mysql --ssl-ca=cacert.pem \
--ssl-cert=client-cert.pem \
--ssl-key=client-key.pem
In other words, the options are similar to those used for the
server. Note that the Certificate Authority certificate has to
be the same.
A client can determine whether the current connection with the
server uses SSL by checking the value of the
Ssl_cipher status variable.
The value of Ssl_cipher is
nonempty if SSL is used, and empty otherwise. For example:
mysql> SHOW STATUS LIKE 'Ssl_cipher';
+---------------+--------------------+
| Variable_name | Value |
+---------------+--------------------+
| Ssl_cipher | DHE-RSA-AES256-SHA |
+---------------+--------------------+
For the mysql client, you can use the
STATUS or \s command and
check the SSL line:
mysql> \s
...
SSL: Not in use
...
Or:
mysql> \s
...
SSL: Cipher in use is DHE-RSA-AES256-SHA
...
To establish a secure connection from within an application
program, use the mysql_ssl_set()
C API function to set the appropriate certificate options before
calling mysql_real_connect().
See Section 20.8.3.67, “mysql_ssl_set()”. After the connection is
established, you can use
mysql_get_ssl_cipher() to
determine whether SSL is in use. A non-NULL
return value indicates a secure connection and names the SSL
cipher used for encryption. A NULL return
value indicates that SSL is not being used. See
Section 20.8.3.33, “mysql_get_ssl_cipher()”.
5.5.7.3. SSL Command Options
The following list describes options that are used for
specifying the use of SSL, certificate files, and key files.
They can be given on the command line or in an option file.
These options are not available unless MySQL has been built with
SSL support. See Section 5.5.7.2, “Using SSL Connections”. (There are
also --master-ssl* options that can be used for
setting up a secure connection from a slave replication server
to a master server; see Section 16.1.2, “Replication and Binary Logging Options and Variables”.)
Table 5.7. SSL Option/Variable Summary | Name | Cmd-Line | Option file | System Var | Status Var | Var Scope | Dynamic |
|---|
| have_openssl | | | Yes | | Global | No | | have_ssl | | | Yes | | Global | No | | skip-ssl | Yes | Yes | | | | | | ssl | Yes | Yes | | | | | | ssl-ca | Yes | Yes | | | Global | No | | - Variable: ssl_ca | | | Yes | | Global | No | | ssl-capath | Yes | Yes | | | Global | No | | - Variable: ssl_capath | | | Yes | | Global | No | | ssl-cert | Yes | Yes | | | Global | No | | - Variable: ssl_cert | | | Yes | | Global | No | | ssl-cipher | Yes | Yes | | | Global | No | | - Variable: ssl_cipher | | | Yes | | Global | No | | ssl-key | Yes | Yes | | | Global | No | | - Variable: ssl_key | | | Yes | | Global | No |
--ssl
For the server, this option specifies that the server allows
SSL connections. For a client program, it allows the client
to connect to the server using SSL. This option is not
sufficient in itself to cause an SSL connection to be used.
You must also specify the
--ssl-ca option, and
possibly the --ssl-cert and
--ssl-key options.
This option is more often used in its opposite form to
override any other SSL options and indicate that SSL should
not be used. To do this, specify the
option as
--skip-ssl
or --ssl=0.
Note that use of --ssl does
not require an SSL connection. For
example, if the server or client is compiled without SSL
support, a normal unencrypted connection is used.
The secure way to require use of an SSL connection is to
create an account on the server that includes a
REQUIRE SSL clause in the
GRANT statement. Then use
that account to connect to the server, where both the server
and the client have SSL support enabled.
The REQUIRE clause allows other
SSL-related restrictions as well. The description of
REQUIRE in Section 12.5.1.3, “GRANT Syntax”,
provides additional detail about which SSL command options
may or must be specified by clients that connect using
accounts that are created using the various
REQUIRE options.
--ssl-ca=file_name
The path to a file that contains a list of trusted SSL CAs.
--ssl-capath=directory_name
The path to a directory that contains trusted SSL CA
certificates in PEM format.
--ssl-cert=file_name
The name of the SSL certificate file to use for establishing
a secure connection.
--ssl-cipher=cipher_list
A list of allowable ciphers to use for SSL encryption. For
greatest portability, cipher_list
should be a list of one or more cipher names, separated by
colons. Examples:
--ssl-cipher=AES128-SHA
--ssl-cipher=DHE-RSA-AES256-SHA:AES128-SHA
This format is understood both by OpenSSL and yaSSL. OpenSSL
supports a more flexible syntax for specifying ciphers, as
described in the OpenSSL documentation at
http://www.openssl.org/docs/apps/ciphers.html.
However, this extended syntax will fail if used with a MySQL
installation compiled against yaSSL.
If no cipher in the list is supported, SSL connections will
not work.
--ssl-key=file_name
The name of the SSL key file to use for establishing a
secure connection.
--ssl-verify-server-cert
This option is available for client programs only, not the
server. It causes the server's Common Name value in the
certificate that the server sends to the client to be
verified against the host name that the client uses for
connecting to the server, and the connection is rejected if
there is a mismatch. This feature can be used to prevent
man-in-the-middle attacks. Verification is disabled by
default. This option was added in MySQL 5.0.23.
As of MySQL 5.0.40, if you use SSL when establishing a client
connection, you can tell the client not to authenticate the
server certificate by specifying neither
--ssl-ca nor
--ssl-capath. The server still
verifies the client according to any applicable requirements
established via GRANT statements
for the client, and it still uses any
--ssl-ca/--ssl-capath
values that were passed to server at startup time.
5.5.7.4. Setting Up SSL Certificates for MySQL
This section demonstrates how to set up SSL certificate and key
files for use by MySQL servers and clients. The first example
shows a simplified procedure such as you might use from the
command line. The second shows a script that contains more
detail. The first two examples are intended for use on Unix and
both use the openssl command that is part of
OpenSSL. The third example describes how to set up SSL files on
Windows.
Following the third example, instructions are given for using
the files to test SSL connections. You can also use the files as
described in Section 5.5.7.2, “Using SSL Connections”.
Example 1: Creating SSL files from the
command line on Unix
The following example shows a set of commands to create MySQL
server and client certificate and key files. You will need to
respond to several prompts by the openssl
commands. For testing, you can press Enter to all prompts. For
production use, you should provide nonempty responses.
# Create clean environment
shell> rm -rf newcerts
shell> mkdir newcerts && cd newcerts
# Create CA certificate
shell> openssl genrsa 2048 > ca-key.pem
shell> openssl req -new -x509 -nodes -days 1000 \
-key ca-key.pem > ca-cert.pem
# Create server certificate
shell> openssl req -newkey rsa:2048 -days 1000 \
-nodes -keyout server-key.pem > server-req.pem
shell> openssl x509 -req -in server-req.pem -days 1000 \
-CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > server-cert.pem
# Create client certificate
shell> openssl req -newkey rsa:2048 -days 1000 \
-nodes -keyout client-key.pem > client-req.pem
shell> openssl x509 -req -in client-req.pem -days 1000 \
-CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > client-cert.pem
Example 2: Creating SSL files using a
script on Unix
Here is an example script that shows how to set up SSL
certificates for MySQL:
DIR=`pwd`/openssl
PRIV=$DIR/private
mkdir $DIR $PRIV $DIR/newcerts
cp /usr/share/ssl/openssl.cnf $DIR
replace ./demoCA $DIR -- $DIR/openssl.cnf
# Create necessary files: $database, $serial and $new_certs_dir
# directory (optional)
touch $DIR/index.txt
echo "01" > $DIR/serial
#
# Generation of Certificate Authority(CA)
#
openssl req -new -x509 -keyout $PRIV/cakey.pem -out $DIR/cacert.pem \
-days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ................++++++
# .........++++++
# writing new private key to '/home/monty/openssl/private/cakey.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL admin
# Email Address []:
#
# Create server request and key
#
openssl req -new -keyout $DIR/server-key.pem -out \
$DIR/server-req.pem -days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ..++++++
# ..........++++++
# writing new private key to '/home/monty/openssl/server-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL server
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:
#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/server-key.pem -out $DIR/server-key.pem
#
# Sign server cert
#
openssl ca -policy policy_anything -out $DIR/server-cert.pem \
-config $DIR/openssl.cnf -infiles $DIR/server-req.pem
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName :PRINTABLE:'FI'
# organizationName :PRINTABLE:'MySQL AB'
# commonName :PRINTABLE:'MySQL admin'
# Certificate is to be certified until Sep 13 14:22:46 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated
#
# Create client request and key
#
openssl req -new -keyout $DIR/client-key.pem -out \
$DIR/client-req.pem -days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# .....................................++++++
# .............................................++++++
# writing new private key to '/home/monty/openssl/client-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL user
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:
#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/client-key.pem -out $DIR/client-key.pem
#
# Sign client cert
#
openssl ca -policy policy_anything -out $DIR/client-cert.pem \
-config $DIR/openssl.cnf -infiles $DIR/client-req.pem
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName :PRINTABLE:'FI'
# organizationName :PRINTABLE:'MySQL AB'
# commonName :PRINTABLE:'MySQL user'
# Certificate is to be certified until Sep 13 16:45:17 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated
#
# Create a my.cnf file that you can use to test the certificates
#
cnf=""
cnf="$cnf [client]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/client-cert.pem"
cnf="$cnf ssl-key=$DIR/client-key.pem"
cnf="$cnf [mysqld]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/server-cert.pem"
cnf="$cnf ssl-key=$DIR/server-key.pem"
echo $cnf | replace " " '
' > $DIR/my.cnf
Example 3: Creating SSL files on
Windows
Download OpenSSL for Windows. An overview of available packages
can be seen here:
http://www.slproweb.com/products/Win32OpenSSL.html
Choose of the following packages, depending on your architecture
(32-bit or 64-bit):
if a message occurs during setup indicating
'...critical component is missing: Microsoft Visual C++
2008 Redistributables', cancel the setup and download
one of the following packages as well, again depending on your
architecture (32-bit or 64-bit):
After installing the additional package, restart the OpenSSL
setup.
During installation, leave the default
C:\OpenSSL as the install path, and also
leave the default option 'Copy OpenSSL DLL files to the
Windows system directory' selected.
When the installation has finished, add
C:\OpenSSL\bin to the Windows System Path
variable of your server:
On the Windows desktop, right-click on the My
Computer icon, and select
.
Next select the tab from
the menu that appears,
and click the Environment Variables
button.
Under System Variables, select
, and then click the
Edit button. The dialogue should appear.
Add ';C:\OpenSSL\bin' to the end (notice
the semicolon).
Press OK 3 times.
Check that OpenSSL was correctly integrated into the Path
variable by opening a new command console
(Start>Run>cmd.exe) and verifying
that OpenSSL is available:
Microsoft Windows [Version ...]
Copyright (c) 2006 Microsoft Corporation. All rights reserved.
C:\Windows\system32>cd \
C:\>openssl
OpenSSL> exit <<< If you see the OpenSSL prompt, installation was successful.
C:\>
Depending on your version of Windows, the preceding instructions
might be slightly different.
After OpenSSL has been installed, use the instructions from
Example 1 (shown earlier in this section), with the following
changes:
Change the follow Unix commands:
# Create clean environment
shell> rm -rf newcerts
shell> mkdir newcerts && cd newcerts
On Windows, use these commands instead:
# Create clean environment
shell> md c:\newcerts
shell> cd c:\newcerts
When a '\' character is shown at the end
of a command line, this '\' character
must be removed and the command lines entered all on a
single line.
For references to my.cnf option files,
substitute my.ini instead.
Testing SSL connections
To test SSL connections, start the server as follows, where
$DIR is the path name to the directory where
the sample my.cnf option file is located:
shell> mysqld --defaults-file=$DIR/my.cnf &
Then invoke a client program using the same option file:
shell> mysql --defaults-file=$DIR/my.cnf
If you have a MySQL source distribution, you can also test your
setup by modifying the preceding my.cnf
file to refer to the demonstration certificate and key files in
the SSL directory of the distribution.
5.5.8. Connecting to MySQL Remotely from Windows with SSH
This section describes how to get a secure connection to a remote
MySQL server with SSH. The information was provided by David
Carlson <dcarlson@mplcomm.com>.
Install an SSH client on your Windows machine. As a user, the
best nonfree one I have found is from
SecureCRT from
http://www.vandyke.com/. Another option is
f-secure from
http://www.f-secure.com/. You can also find
some free ones on Google at
http://directory.google.com/Top/Computers/Internet/Protocols/SSH/Clients/Windows/.
Start your Windows SSH client. Set Host_Name =
yourmysqlserver_URL_or_IP.
Set
userid=your_userid
to log in to your server. This userid value
might not be the same as the user name of your MySQL account.
Set up port forwarding. Either do a remote forward (Set
local_port: 3306, remote_host:
yourmysqlservername_or_ip,
remote_port: 3306 ) or a local forward (Set
port: 3306, host:
localhost, remote port: 3306).
Save everything, otherwise you will have to redo it the next
time.
Log in to your server with the SSH session you just created.
On your Windows machine, start some ODBC application (such as
Access).
Create a new file in Windows and link to MySQL using the ODBC
driver the same way you normally do, except type in
localhost for the MySQL host server, not
yourmysqlservername.
At this point, you should have an ODBC connection to MySQL,
encrypted using SSH.
5.5.9. Auditing MySQL Account Activity
Applications can use the following guidelines to perform auditing
that ties database activity to MySQL accounts.
MySQL accounts correspond to rows in the
mysql.user table. When a client connects
successfully, the server authenticates the client to a particular
row in this table. The User and
Host column values in this row uniquely
identify the account and correspond to the
'user_name'@'host_name'
format in which account names are written in SQL statements.
The account used to authenticate a client determines which
privileges the client has. Normally, the
CURRENT_USER() function can be
invoked to determine which account this is for the client user.
Its value is constructed from the User and
Host columns of the user
table row for the account.
However, there are circumstances under which the
CURRENT_USER() value corresponds
not to the client user but to a different account. This occurs in
contexts when privilege checking is not based the client's
account:
Stored routines (procedures and functions) defined with the
SQL SECURITY DEFINER characteristic.
Views defined with the SQL SECURITY DEFINER
characteristic (as of MySQL 5.0.24).
Triggers (as of MySQL 5.0.17).
In those contexts, privilege checking is done against the
DEFINER account and
CURRENT_USER() refers to that
account, not to the account for the client who invoked the stored
routine or view or who caused the trigger to activate. To
determine the invoking user, you can call the
USER() function, which returns a
value indicating the actual user name provided by the client and
the host from which the client connected. However, this value does
not necessarily correspond directly to an account in the
user table, because the
USER() value never contains
wildcards, whereas account values (as returned by
CURRENT_USER()) may contain user
name and host name wildcards.
For example, a blank user name matches any user, so an account of
''@'localhost' enables clients to connect as an
anonymous user from the local host with any user name. If this
case, if a client connects as user1 from the
local host, USER() and
CURRENT_USER() return different
values:
mysql> SELECT USER(), CURRENT_USER();
+-----------------+----------------+
| USER() | CURRENT_USER() |
+-----------------+----------------+
| user1@localhost | @localhost |
+-----------------+----------------+
The host name part of an account can contain wildcards, too. If
the host name contains a '%' or
'_' pattern character or uses netmask notation,
the account can be used for clients connecting from multiple hosts
and the CURRENT_USER() value will
not indicate which one. For example, the account
'user2'@'%.example.com' can be used by
user2 to connect from any host in the
example.com domain. If user2
connects from remote.example.com,
USER() and
CURRENT_USER() return different
values:
mysql> SELECT USER(), CURRENT_USER();
+--------------------------+---------------------+
| USER() | CURRENT_USER() |
+--------------------------+---------------------+
| user2@remote.example.com | user2@%.example.com |
+--------------------------+---------------------+
If an application must invoke
USER() for user auditing (for
example, if it does auditing from within triggers) but must also
be able to associate the USER()
value with an account in the user table, it is
necessary to avoid accounts that contain wildcards in the
User or Host column.
Specifically, do not allow User to be empty
(which creates an anonymous-user account), and do not allow
pattern characters or netmask notation in Host
values. All accounts must have a nonempty User
value and literal Host value.
With respect to the previous examples, the
''@'localhost' and
'user2'@'%.example.com' accounts should be
changed not to use wildcards:
RENAME USER ''@'localhost' TO 'user1'@'localhost';
RENAME USER 'user2'@'%.example.com' TO 'user2'@'remote.example.com';
If user2 must be able to connect from several
hosts in the example.com domain, there should
be a separate account for each host.
To extract the user name or host name part from a
CURRENT_USER() or
USER() value, use the
SUBSTRING() function:
mysql> SELECT SUBSTRING_INDEX(CURRENT_USER(),'@',1);
+---------------------------------------+
| SUBSTRING_INDEX(CURRENT_USER(),'@',1) |
+---------------------------------------+
| user1 |
+---------------------------------------+
mysql> SELECT SUBSTRING_INDEX(CURRENT_USER(),'@',-1);
+----------------------------------------+
| SUBSTRING_INDEX(CURRENT_USER(),'@',-1) |
+----------------------------------------+
| localhost |
+----------------------------------------+
5.6. Running Multiple MySQL Servers on the Same Machine
In some cases, you might want to run multiple
mysqld servers on the same machine. You might
want to test a new MySQL release while leaving your existing
production setup undisturbed. Or you might want to give different
users access to different mysqld servers that
they manage themselves. (For example, you might be an Internet
Service Provider that wants to provide independent MySQL
installations for different customers.)
To run multiple servers on a single machine, each server must have
unique values for several operating parameters. These can be set on
the command line or in option files. See
Section 4.2.3, “Specifying Program Options”.
At least the following options must be different for each server:
--port=port_num
--port controls the port number
for TCP/IP connections. (Alternatively, if the host has multiple
network addresses, you can use
--bind-address to cause different
servers to listen to different interfaces.)
--socket=path
--socket controls the Unix socket
file path on Unix and the name of the named pipe on Windows. On
Windows, it is necessary to specify distinct pipe names only for
those servers that support named-pipe connections.
--shared-memory-base-name=name
This option currently is used only on Windows. It designates the
shared-memory name used by a Windows server to allow clients to
connect via shared memory. It is necessary to specify distinct
shared-memory names only for those servers that support
shared-memory connections.
--pid-file=file_name
This option is used only on Unix. It indicates the path name of
the file in which the server writes its process ID.
If you use the following log file options, they must be different
for each server:
Section 5.2.5, “Server Log Maintenance”, discusses the log file
options further.
For better performance, you can specify the following options
differently for each server, to spread the load between several
physical disks:
Having different temporary directories also makes it easier to
determine which MySQL server created any given temporary file.
With very limited exceptions, each server should use a different
data directory, which is specified using the
--datadir=path
option.
Warning
Normally, you should never have two servers that update data in
the same databases. This may lead to unpleasant surprises if your
operating system does not support fault-free system locking. If
(despite this warning) you run multiple servers using the same
data directory and they have logging enabled, you must use the
appropriate options to specify log file names that are unique to
each server. Otherwise, the servers try to log to the same files.
Please note that this kind of setup only works with
MyISAM and MERGE tables, and
not with any of the other storage engines.
The warning against sharing a data directory among servers also
applies in an NFS environment. Allowing multiple MySQL servers to
access a common data directory over NFS is a very bad
idea.
The primary problem is that NFS is the speed bottleneck. It is
not meant for such use.
Another risk with NFS is that you must devise a way to ensure
that two or more servers do not interfere with each other.
Usually NFS file locking is handled by the
lockd daemon, but at the moment there is no
platform that performs locking 100% reliably in every situation.
Make it easy for yourself: Forget about sharing a data directory
among servers over NFS. A better solution is to have one computer
that contains several CPUs and use an operating system that handles
threads efficiently.
If you have multiple MySQL installations in different locations, you
can specify the base installation directory for each server with the
--basedir=path
option to cause each server to use a different data directory, log
files, and PID file. (The defaults for all these values are
determined relative to the base directory). In that case, the only
other options you need to specify are the
--socket and
--port options. Suppose that you
install different versions of MySQL using tar
file binary distributions. These install in different locations, so
you can start the server for each installation using the command
bin/mysqld_safe under its corresponding base
directory. mysqld_safe determines the proper
--basedir option to pass to
mysqld, and you need specify only the
--socket and
--port options to
mysqld_safe.
As discussed in the following sections, it is possible to start
additional servers by setting environment variables or by specifying
appropriate command-line options. However, if you need to run
multiple servers on a more permanent basis, it is more convenient to
use option files to specify for each server those option values that
must be unique to it. The
--defaults-file option is useful for
this purpose.
5.6.1. Running Multiple Servers on Windows
You can run multiple servers on Windows by starting them manually
from the command line, each with appropriate operating parameters.
You also have the option of installing several servers as Windows
services and running them that way. General instructions for
running MySQL servers from the command line or as services are
given in Section 2.9, “Installing MySQL on Windows”. This section
describes how to make sure that you start each server with
different values for those startup options that must be unique per
server, such as the data directory. These options are described in
Section 5.6, “Running Multiple MySQL Servers on the Same Machine”.
5.6.1.1. Starting Multiple Windows Servers at the Command Line
To start multiple servers manually from the command line, you
can specify the appropriate options on the command line or in an
option file. It is more convenient to place the options in an
option file, but it is necessary to make sure that each server
gets its own set of options. To do this, create an option file
for each server and tell the server the file name with a
--defaults-file option when you
run it.
Suppose that you want to run mysqld on port
3307 with a data directory of C:\mydata1,
and mysqld-debug on port 3308 with a data
directory of C:\mydata2. (To do this, make
sure that before you start the servers, each data directory
exists and has its own copy of the mysql
database that contains the grant tables.) Then create two option
files. For example, create one file named
C:\my-opts1.cnf that looks like this:
[mysqld]
datadir = C:/mydata1
port = 3307
Create a second file named C:\my-opts2.cnf
that looks like this:
[mysqld]
datadir = C:/mydata2
port = 3308
Then start each server with its own option file:
C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
C:\> C:\mysql\bin\mysqld-debug --defaults-file=C:\my-opts2.cnf
Each server starts in the foreground (no new prompt appears
until the server exits later), so you will need to issue those
two commands in separate console windows.
To shut down the servers, you must connect to each using the
appropriate port number:
C:\> C:\mysql\bin\mysqladmin --port=3307 shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 shutdown
Servers configured as just described allow clients to connect
over TCP/IP. If your version of Windows supports named pipes and
you also want to allow named-pipe connections, use the
mysqld-nt or mysqld-debug
server and specify options that enable the named pipe and
specify its name. Each server that supports named-pipe
connections must use a unique pipe name. For example, the
C:\my-opts1.cnf file might be written like
this:
[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1
Then start the server this way:
C:\> C:\mysql\bin\mysqld-nt --defaults-file=C:\my-opts1.cnf
Modify C:\my-opts2.cnf similarly for use by
the second server.
A similar procedure applies for servers that you want to support
shared-memory connections. Enable such connections with the
--shared-memory option and
specify a unique shared-memory name for each server with the
--shared-memory-base-name option.
5.6.1.2. Starting Multiple Windows Servers as Services
A MySQL server can run as a Windows service. The procedures for
installing, controlling, and removing a single MySQL service are
described in Section 2.9.11, “Starting MySQL as a Windows Service”.
You can also install multiple MySQL servers as services. In this
case, you must make sure that each server uses a different
service name in addition to all the other parameters that must
be unique for each server.
For the following instructions, assume that you want to run the
mysqld-nt server from two different versions
of MySQL that are installed at
C:\mysql-4.1.8 and
C:\mysql-5.0.91, respectively.
(This might be the case if you're running 4.1.8 as your
production server, but also want to conduct tests using
5.0.91.)
The following principles apply when installing a MySQL service
with the --install or
--install-manual option:
If you specify no service name, the server uses the default
service name of MySQL and the server
reads options from the [mysqld] group in
the standard option files.
If you specify a service name after the
--install option, the server ignores the
[mysqld] option group and instead reads
options from the group that has the same name as the
service. The server reads options from the standard option
files.
If you specify a
--defaults-file option after
the service name, the server ignores the standard option
files and reads options only from the
[mysqld] group of the named file.
Note
In MySQL 5.0, all servers read the
[mysqld] group if they read the standard
option files, whether installed using the default service name
(MySQL) or another service name. This
allows you to use the [mysqld] group for
options that should be used by all MySQL services, and an
option group named after each service for use by the server
installed with that service name.
Based on the preceding information, you have several ways to set
up multiple services. The following instructions describe some
examples. Before trying any of them, be sure that you shut down
and remove any existing MySQL services first.
Approach 1: Specify the
options for all services in one of the standard option
files. To do this, use a different service name for each
server. Suppose that you want to run the 4.1.8
mysqld-nt using the service name of
mysqld1 and the 5.0.91
mysqld-nt using the service name
mysqld2. In this case, you can use the
[mysqld1] group for 4.1.8 and the
[mysqld2] group for 5.0.91.
For example, you can set up C:\my.cnf
like this:
# options for mysqld1 service
[mysqld1]
basedir = C:/mysql-4.1.8
port = 3307
enable-named-pipe
socket = mypipe1
# options for mysqld2 service
[mysqld2]
basedir = C:/mysql-5.0.91
port = 3308
enable-named-pipe
socket = mypipe2
Install the services as follows, using the full server path
names to ensure that Windows registers the correct
executable program for each service:
C:\> C:\mysql-4.1.8\bin\mysqld-nt --install mysqld1
C:\> C:\mysql-5.0.91\bin\mysqld-nt --install mysqld2
To start the services, use the services manager, or use
NET START with the appropriate service
names:
C:\> NET START mysqld1
C:\> NET START mysqld2
To stop the services, use the services manager, or use
NET STOP with the appropriate service
names:
C:\> NET STOP mysqld1
C:\> NET STOP mysqld2
Approach 2: Specify options
for each server in separate files and use
--defaults-file when you
install the services to tell each server what file to use.
In this case, each file should list options using a
[mysqld] group.
With this approach, to specify options for the 4.1.8
mysqld-nt, create a file
C:\my-opts1.cnf that looks like this:
[mysqld]
basedir = C:/mysql-4.1.8
port = 3307
enable-named-pipe
socket = mypipe1
For the 5.0.91 mysqld-nt,
create a file C:\my-opts2.cnf that
looks like this:
[mysqld]
basedir = C:/mysql-5.0.91
port = 3308
enable-named-pipe
socket = mypipe2
Install the services as follows (enter each command on a
single line):
C:\> C:\mysql-4.1.8\bin\mysqld-nt --install mysqld1
--defaults-file=C:\my-opts1.cnf
C:\> C:\mysql-5.0.91\bin\mysqld-nt --install mysqld2
--defaults-file=C:\my-opts2.cnf
To use a --defaults-file
option when you install a MySQL server as a service, you
must precede the option with the service name.
After installing the services, start and stop them the same
way as in the preceding example.
To remove multiple services, use mysqld
--remove for each one, specifying a service name
following the --remove option. If the service
name is the default (MySQL), you can omit it.
5.6.2. Running Multiple Servers on Unix
The easiest way is to run multiple servers on Unix is to compile
them with different TCP/IP ports and Unix socket files so that
each one is listening on different network interfaces. Compiling
in different base directories for each installation also results
automatically in a separate, compiled-in data directory, log file,
and PID file location for each server.
Assume that an existing 4.1.8 server is configured for the default
TCP/IP port number (3306) and Unix socket file
(/tmp/mysql.sock). To configure a new
5.0.91 server to have different operating parameters,
use a configure command something like this:
shell> ./configure --with-tcp-port=port_number \
--with-unix-socket-path=file_name \
--prefix=/usr/local/mysql-5.0.91
Here, port_number and
file_name must be different from the
default TCP/IP port number and Unix socket file path name, and the
--prefix value should specify an
installation directory different from the one under which the
existing MySQL installation is located.
If you have a MySQL server listening on a given port number, you
can use the following command to find out what operating
parameters it is using for several important configurable
variables, including the base directory and Unix socket file name:
shell> mysqladmin --host=host_name --port=port_number variables
With the information displayed by that command, you can tell what
option values not to use when configuring an
additional server.
Note that if you specify localhost as a host
name, mysqladmin defaults to using a Unix
socket file connection rather than TCP/IP. You can explicitly
specify the connection protocol to use by using the
--protocol={TCP|SOCKET|PIPE|MEMORY}
option.
You don't have to compile a new MySQL server just to start with a
different Unix socket file and TCP/IP port number. It is also
possible to use the same server binary and start each invocation
of it with different parameter values at runtime. One way to do so
is by using command-line options:
shell> mysqld_safe --socket=file_name --port=port_number
To start a second server, provide different
--socket and
--port option values, and pass a
--datadir=path
option to mysqld_safe so that the server uses a
different data directory.
Another way to achieve a similar effect is to use environment
variables to set the Unix socket file name and TCP/IP port number:
shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysql_install_db --user=mysql
shell> mysqld_safe --datadir=/path/to/datadir &
This is a quick way of starting a second server to use for
testing. The nice thing about this method is that the environment
variable settings apply to any client programs that you invoke
from the same shell. Thus, connections for those clients are
automatically directed to the second server.
Section 2.20, “Environment Variables”, includes a list of other
environment variables you can use to affect
mysqld.
For automatic server execution, the startup script that is
executed at boot time should execute the following command once
for each server with an appropriate option file path for each
command:
shell> mysqld_safe --defaults-file=file_name
Each option file should contain option values specific to a given
server.
On Unix, the mysqld_multi script is another way
to start multiple servers. See Section 4.3.4, “mysqld_multi — Manage Multiple MySQL Servers”.
5.6.3. Using Client Programs in a Multiple-Server Environment
To connect with a client program to a MySQL server that is
listening to different network interfaces from those compiled into
your client, you can use one of the following methods:
Start the client with
--host=host_name
--port=port_number
to connect via TCP/IP to a remote server, with
--host=127.0.0.1
--port=port_number
to connect via TCP/IP to a local server, or with
--host=localhost
--socket=file_name
to connect to a local server via a Unix socket file or a
Windows named pipe.
Start the client with
--protocol=TCP to connect via
TCP/IP, --protocol=SOCKET to
connect via a Unix socket file,
--protocol=PIPE to connect via
a named pipe, or
--protocol=MEMORY to connect
via shared memory. For TCP/IP connections, you may also need
to specify --host and
--port options. For the other
types of connections, you may need to specify a
--socket option to specify a
Unix socket file or Windows named-pipe name, or a
--shared-memory-base-name
option to specify the shared-memory name. Shared-memory
connections are supported only on Windows.
On Unix, set the MYSQL_UNIX_PORT and
MYSQL_TCP_PORT environment variables to
point to the Unix socket file and TCP/IP port number before
you start your clients. If you normally use a specific socket
file or port number, you can place commands to set these
environment variables in your .login file
so that they apply each time you log in. See
Section 2.20, “Environment Variables”.
Specify the default Unix socket file and TCP/IP port number in
the [client] group of an option file. For
example, you can use C:\my.cnf on
Windows, or the .my.cnf file in your home
directory on Unix. See Section 4.2.3.3, “Using Option Files”.
In a C program, you can specify the socket file or port number
arguments in the
mysql_real_connect() call. You
can also have the program read option files by calling
mysql_options(). See
Section 20.8.3, “C API Function Descriptions”.
If you are using the Perl DBD::mysql
module, you can read options from MySQL option files. For
example:
$dsn = "DBI:mysql:test;mysql_read_default_group=client;"
. "mysql_read_default_file=/usr/local/mysql/data/my.cnf";
$dbh = DBI->connect($dsn, $user, $password);
See Section 20.10, “MySQL Perl API”.
Other programming interfaces may provide similar capabilities
for reading option files.
MySQL Enterprise
Subscribers to MySQL Enterprise will find additional information
on running multiple MySQL servers on one machine in the MySQL
Enterprise Knowledge Base article found at
https://kb.mysql.com/view.php?id=4926.
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