From Linux Man Pages
mysqld - the MySQL server
mysqld [options]
DESCRIPTION
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
MYSQLD COMMAND OPTIONS
When you start the mysqld server, you can specify program options using any of the methods described in
Section 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 3.2, "Using Option
Files".
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, "Security-Related mysqld Options".
� SSL-related options: See Section 7.7.3, "SSL Command Options".
� Binary log control options: See Section 10.3, "The Binary Log".
� Replication-related options: See Section 8, "Replication Startup Options".
� Options specific to particular storage engines: See Section 1.1, "MyISAM Startup Options", Section 5.3, "BDB
Startup Options", Section 2.4, "InnoDB Startup Options and System Variables", and Section 6.5.1, "MySQL
Cluster-Related Command Options for mysqld".
You can also set the values of server system variables by using variable names as options, as described later in
this section.
� --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 2.4.6, "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 9.3, "Running MySQL in ANSI Mode", and the section called "THE
SERVER SQL MODE".
� --basedir=path, -b path
The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
� --bind-address=IP
The IP address to bind to.
� --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.
� --character-sets-dir=path
The directory where character sets are installed. See Section 9.1, "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 filesystem 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 9.1, "The Character Set Used for Data and
Sorting".
� --chroot=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 9.1, "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. For some systems, you must also specify the --core-file-size option to
mysqld_safe. See mysqld_safe(1). Note that on some systems, such as Solaris, you do not get a core file if you
are also using the --user option.
� --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.
The debug_options string often is �d:t:o,file_name'. The default is �d:t:i:o,mysqld.trace'. See Section 1.2,
"Creating Trace Files".
� --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 9.1, "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 9.1, "The Character Set Used for Data and Sorting".
� --default-storage-engine=type
Set the default storage engine (table type) for tables. See Chapter 14, Storage Engines and Table Types.
� --default-table-type=type
This option is a 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 5.2,
"Tuning Server Parameters", and Section 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 applies only on Windows NT, 2000, XP, and 2003 systems, and can be
used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.
� --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.
Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you must ensure
that the following conditions are satisfied:
� You should not use the query cache for queries that use tables that are updated by another process.
� You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables.
The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF and
--query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of
the preceding options.)
� --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 4.2, "What to Do If MySQL Keeps Crashing".
� --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.
� --innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See Section 10.3, "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 2.4, "InnoDB Startup Options and System Variables".
� --language=lang_name, -L lang_name
Return client error messages in the given language. lang_name can be given as the language name or as the
full pathname to the directory where the language files are installed. See Section 9.2, "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 OS. 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 pages support (which is called HugeTLB in
Linux). We have plans to extend this support to FreeBSD, Solaris and possibly other platforms.
Before large pages can be used on Linux, it is necessary to configure the HugeTLB memory pool. For reference,
consult the hugetlbpage.txt file in the Linux kernel source.
This option 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 10.2, "The General Query
Log". If you omit the filename, MySQL uses host_name.log as the filename.
� --log-bin[=base_name]
Enable binary logging. The server logs all statements that change data to the binary log, which is used for
backup and replication. See Section 10.3, "The Binary Log".
The option value, if given, is the basename for the log sequence. The server creates binary log files in
sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see
Section 8.1, "Open Issues in MySQL", for the reason). Otherwise, MySQL uses host_name-bin as the basename.
� --log-bin-index[=file_name]
The index file for binary log filenames. See Section 10.3, "The Binary Log". If you omit the filename, and if
you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the filename.
� --log-bin-trust-function-creators[={0|1}]
With no argument or an argument of 1, this option sets the log_bin_trust_function_creators system variable to
1. With an argument of 0, this option sets the system variable to 0. log_bin_trust_function_creators affects
how MySQL enforces restrictions on stored function creation. See Section 4, "Binary Logging of Stored Routines
and Triggers".
This option was added in MySQL 5.0.16.
� --log-bin-trust-routine-creators[={0|1}]
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 and sets the log_bin_trust_routine_creators system variable. As of
5.0.16, this option is deprecated. It is recognized for backward compatibility but its use results in a
warning.
This option was added in MySQL 5.0.6.
� --log-error[=file_name]
Log errors and startup messages to this file. See Section 10.1, "The Error Log". If you omit the filename,
MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .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 username 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 --log-slow-queries, queries that do not use indexes are logged to the slow
query log. See Section 10.4, "The Slow Query Log".
� --log-short-format
Log less information to the update log, binary update log, and slow query log, if they have been activated.
For example, the username and timestamp are not logged for queries.
� --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 10.4,
"The Slow Query Log". See the descriptions of the --log-long-format and --log-short-format options for
details.
� --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. Aborted connections are not logged to the error
log unless the value is greater than 1. See Section 2.10, "Communication Errors and Aborted Connections".
� --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. See Section 3.2, "Table Locking Issues".
� --memlock
Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() system
call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note
that use of this option requires that you run the server as root, which is normally not a good idea for
security reasons. See Section 5.5, "How to Run MySQL as a Normal User".
� --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. You can also use a value of
"" to disable this option. If this option is used, 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: OptionDescriptionDEFAULTThe same as not giving any option
to --myisam-recover.BACKUPIf the data file was changed during recovery, save a backup of the
tbl_name.MYD
file as
tbl_name-datetime.BAK.FORCERun recovery even if we would lose more than one row from the
.MYD file.QUICKDon't check the rows in the table if there aren't any delete
blocks.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 1.1, "MyISAM Startup Options".
� --ndb-connectstring=connect_string
When using the NDB storage engine, it is possible to point out the management server that distributes the
cluster configuration by setting the connect string option. See Section 4.4.2, "The MySQL Cluster
connectstring", for syntax.
� --ndbcluster
If the binary includes support for the NDB Cluster storage engine, this option enables the engine, which is
disabled by default. See Chapter 15, MySQL Cluster.
� --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 6.9, "Password Hashing as of
MySQL 4.1".
� --one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built with
debugging enabled. See Section 1, "Debugging a MySQL Server".
� --open-files-limit=count
Change the number of file descriptors available to mysqld. If this option is not set or is set to 0, mysqld
uses the value to reserve file descriptors with setrlimit(). If the value is 0, mysqld reserves
max_connections�5 or max_connections + table_open_cache�2 files (whichever is larger). You should try
increasing this value if mysqld gives you the error Too many open files.
� --pid-file=path
The pathname of the process ID file. 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 6.3, "Privileges Provided by MySQL".
� --safe-user-create
If this option is enabled, a user cannot create new MySQL users by using the GRANT statement, if the user
doesn't have the INSERT privilege for the mysql.user table or any column in the table.
� --secure-auth
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords.
� --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 3.3, "Concurrent Inserts".
� --skip-external-locking
Do not use external locking (system locking). With external locking disabled, you must shut down the server to
use myisamchk. (See Section 4.3, "MySQL Stability".) To avoid this requirement, use the CHECK TABLE and REPAIR
TABLE statements to check and repair MyISAM tables.
External locking has been disabled by default since MySQL 4.0.
� --skip-grant-tables
This option causes the server not to use 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).
� --skip-host-cache
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every
time a client connects. See Section 5.6, "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 hostnames 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 5.6, "How MySQL Uses DNS".
� --skip-ndbcluster
Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster
storage engine support; the server allocates memory and other resources for this storage engine only if the
--ndbcluster option is given explicitly. See Section 4.3, "Quick Test Setup of MySQL Cluster", for an example
of usage.
� --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 5.6, "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 7.7.3, "SSL Command Options".
� --standalone
Available on Windows NT-based systems only; 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:
� On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by
creating a db_name.sym file that contains the path to the real directory. See Section 6.1.3, "Using
Symbolic Links for Databases on Windows".
� On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another
directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete
or rename the table, the files that its symbolic links point to also are deleted or renamed. See
Section 6.1.2, "Using Symbolic Links for Tables on 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 Section 1, "Debugging a MySQL Server".
� --skip-thread-priority
Disable using thread priorities for faster response time.
� --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. 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 the section called "THE SERVER SQL MODE".
� --sysdate-is-now
As of MySQL 5.0.13, 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 5, "Date and Time Functions" and for SET TIMESTAMP in
Section 5.3, "SET Syntax".
This option was added in MySQL 5.0.20.
� --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 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 filesystem or to a directory that
is cleared when the server host restarts. For more information about the storage location of temporary files,
see Section 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 5.1, "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.
� --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 5.3,
"SET Syntax".
the section called "SERVER SYSTEM VARIABLES", provides a full description for all variables, and additional
information for setting them at server startup and runtime. Section 5.2, "Tuning Server Parameters", includes
information on optimizing the server by tuning system variables.
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 MySQL 3.23, 4.0,
4.1 Reference Manual.
For additional system variable information, see these sections:
� the section called "USING SYSTEM VARIABLES", discusses the syntax for setting and displaying system variable
values.
� the section called "Dynamic System Variables", lists the variables that can be set at runtime.
� Information on tuning sytem variables can be found in Section 5.2, "Tuning Server Parameters".
� Section 2.4, "InnoDB Startup Options and System Variables", lists InnoDB system 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.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
� auto_increment_increment
auto_increment_increment and auto_increment_offset are intended for use with master-to-master replication, and
can be used to control the operation of AUTO_INCREMENT columns. Both variables can be set globally or locally,
and each can assume an integer value between 1 and 65,535 inclusive. Setting the value of either of these two
variables to 0 causes its value to be set to 1 instead. Attempting to set the value of either of these two
variables to an integer greater than 65,535 or less than 0 causes its value to be set to 65,535 instead.
Attempting to set the value of auto_increment_increment or auto_increment_offset to a non-integer value gives
rise to an error, and the actual value of the variable remains unchanged.
These two variables affect AUTO_INCREMENT column behavior as follows:
� auto_increment_increment controls the interval between successive column values. For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc1
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.04 sec)
mysql> SET @@auto_increment_increment=10;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.01 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
(Note how SHOW VARIABLES is used here to obtain the current values for these variables.)
� auto_increment_offset determines the starting point for the AUTO_INCREMENT column value. Consider the
following, assuming that these statements are executed during the same session as the example given in the
description for auto_increment_increment:
mysql> SET @@auto_increment_offset=5;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc2
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.06 sec)
mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc2;
+-----+
| col |
+-----+
| 5 |
| 15 |
| 25 |
| 35 |
+-----+
4 rows in set (0.02 sec)
If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of
auto_increment_offset is ignored.
Should one or both of these variables be changed and then new rows inserted into a table containing an
AUTO_INCREMENT column, the results may seem counterintuitive because the series of AUTO_INCREMENT values is
calculated without regard to any values already present in the column, and the next value inserted is the
least value in the series that is greater than the maximum existing value in the AUTO_INCREMENT column. In
other words, the series is calculated like so:
auto_increment_offset + N � auto_increment_increment
where N is a positive integer value in the series [1, 2, 3, ...]. For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
| 35 |
| 45 |
| 55 |
| 65 |
+-----+
8 rows in set (0.00 sec)
The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N � 10, that
is, [5, 15, 25, 35, 45, ...]. The greatest value present in the col column prior to the INSERT is 31, and the
next available value in the AUTO_INCREMENT series is 35, so the inserted values for col begin at that point
and the results are as shown for the SELECT query.
It is important to remember that it is not possible to confine the effects of these two variables to a single
table, and thus they do not take the place of the sequences offered by some other database management systems;
these variables control the behavior of all AUTO_INCREMENT columns in all tables on the MySQL server. If one
of these variables is set globally, its effects persist until the global value is changed or overridden by
setting them locally, or until mysqld is restarted. If set locally, the new value affects AUTO_INCREMENT
columns for all tables into which new rows are inserted by the current user for the duration of the session,
unless the values are changed during that session.
The auto_increment_increment variable was added in MySQL 5.0.2. Its default value is 1. See Section 13,
"Auto-Increment in Multiple-Master Replication".
� auto_increment_offset
This variable was introduced in MySQL 5.0.2. Its default value is 1. For particulars, see the description for
auto_increment_increment.
� 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.
� 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 10.3, "The Binary Log".
� 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 non-empty 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.
� 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 filesystem character set. This variable is used to interpret string literals that refer to filenames, such
as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such filenames
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 filenames
are allowed, a different value may be more appropriate. For example, if the system represents filenames using
UTF-8, set character_set_filesytem to �utf8'. This variable was added in MySQL 5.0.19.
� character_set_results
The character set used for returning query results 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 ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have
no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new.
In MySQL 5.0.6, this variable was changed to take three integer values: ValueDescription0Off1(Default) Enables
concurrent insert for MyISAM tables
that don't have holes2Enables concurrent inserts for all MyISAM tables. If
table has a hole and is in use by another thread
the new row will be inserted at end of table. If
table is not in use, MySQL does a normal read lock
and inserts the new row into the hole.See also Section 3.3, "Concurrent Inserts".
� connect_timeout
The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake.
� datadir
The MySQL data directory. This variable can be set with the --datadir option.
� date_format
This variable is not implemented.
� datetime_format
This variable is not implemented.
� default_week_format
The default mode value to use for the WEEK() function. See Section 5, "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. OptionDescriptionOFFDELAY_KEY_WRITE
is ignored.ONMySQL honors any DELAY_KEY_WRITE option specified in
CREATE TABLE statements. This
is the default value.ALLAll new opened tables are treated as if they were created with
the
DELAY_KEY_WRITE option enabled.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 the section
called "\FBMYSQLD\FR COMMAND OPTIONS", and Section 1.1, "MyISAM Startup Options".
Note that enabling external locking with --external-locking offers 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 of precision by which to increase 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
This variable applies to NDB. By default it is 0 (OFF): If you execute a query such as SELECT * FROM t WHERE
mycol = 42, where mycol is a non-indexed column, the query is executed as a full table scan on every NDB node.
Each node 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 and sent to the NDB nodes. Each node
uses the condition to perform the scan, and only sends back to the MySQL server the rows that match the
condition.
This variable was added in MySQL 5.0.3. Before that, the default NDB behavior is the same as for a value of
OFF.
� expire_logs_days
The number of days for automatic binary log removal. The default is 0, which means "no automatic removal."
Possible removals happen at startup and at binary log rotation.
� 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 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 non-zero value, all tables are closed every flush_time seconds to free up resources and
synchronize unflushed data to disk. We recommend that this option be 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 7.1, "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 non-alphanumeric 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. 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 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 ARCHIVE 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_ndbcluster
YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used.
� have_openssl
YES if mysqld supports SSL connections, NO if not.
� 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_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.
� 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.
� init_slave
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL
thread starts. The format of the string is the same as for the init_connect variable.
� innodb_xxx
InnoDB system variables are listed in Section 2.4, "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 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.
� 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. 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.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can
afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common.
However, if you make the value too large (for example, more than 50% of your total memory) your system might
start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching
for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of
other storage engines.
For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 2.16, "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 5.4, "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 4.6, "The MyISAM Key Cache".
� key_cache_age_threshold
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower
values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See
Section 4.6, "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 4.6, "The MyISAM Key
Cache".
� key_cache_division_limit
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the
percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The
default value is 100. See Section 4.6, "The MyISAM Key Cache".
� language
The language used for error messages.
� large_file_support
Whether mysqld was compiled with options for large file support.
� large_pages
Whether large page support is enabled. This variable was added in MySQL 5.0.3.
� license
The type of license the server has.
� local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.4, "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 10.2, "The General Query
Log".
� log_bin
Whether the binary log is enabled. See Section 10.3, "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. See Section 4, "Binary Logging of Stored Routines and Triggers".
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 10.4, "The Slow Query
Log". This variable was added in MySQL 5.0.23.
� log_slave_updates
Whether updates received by a slave server from a master server should be logged to the slave's own binary
log. Binary logging must be enabled on the slave for this variable to have any effect. See Section 8,
"Replication Startup Options".
� log_slow_queries
Whether slow queries should be logged. "Slow" is determined by the value of the long_query_time variable. See
Section 10.4, "The Slow Query Log".
� log_warnings
Whether to produce additional warning messages. It is enabled(1) by default. 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 10.4,
"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 variable previously was named sql_low_priority_updates.
� lower_case_file_system
This variable describes the case sensitivity of filenames on the filesystem where the data directory is
located. OFF means filenames 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 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
filenames (such as Windows or Mac OS X). If this variable is not set at startup and the filesystem on which
the data directory is located does not have case-sensitive filenames, 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.
� max_binlog_cache_size
If a multiple-statement transaction requires more than this amount of memory, the server generates a
Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error.
� max_binlog_size
If a write to the binary log causes the current log file size to exceed the value of this variable, the server
rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more
than 1GB or to less than 4096 bytes. The default value is 1GB.
A transaction is written in one chunk to the binary log, so it is never split between several binary logs.
Therefore, if you have big transactions, you might see binary logs larger than max_binlog_size.
If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well.
� 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.
� max_connections
The number of simultaneous client connections allowed. Increasing this value increases the number of file
descriptors that mysqld requires. See Section 4.8, "How MySQL Opens and Closes Tables", for comments on file
descriptor limits. See also Section 2.6, "Too many connections".
� 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.
� 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.
� 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 2.12,
"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. The default value is 16,382. The allowable range of values is from 0 to
1 milliion. 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. 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 3, "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 5.4.13, "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_tmp_tables
The maximum number of temporary tables a client can keep open at the same time. (This option 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
non-zero 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). This variable was added in MySQL 4.1.2. See Section 2.11, "The table is full".
� myisam_max_extra_sort_file_size (DEPRECATED)
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the
amount specified here, prefer the key cache method. This is mainly used to force long character keys in large
tables to use the slower key cache method to create the index. The value is given in bytes.
Note: This variable 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.
� myisam_recover_options
The value of the --myisam-recover option. See the section called "\FBMYSQLD\FR 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.
� myisam_stats_method
How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM
tables. This variable has two possible values, nulls_equal and nulls_unequal. 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.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query
execution, as described in Section 4.7, "MyISAM Index Statistics Collection".
This variable was added in MySQL 5.0.14. For older versions, the statistics collection method is equivalent to
nulls_equal.
� multi_read_range
Specifies the maximum number of ranges to send to a storage engine during range selects. The default value is
256. Sending multiple ranges to an engine is a feature that can improve the performance of certain selects
dramatically, particularly for NDBCLUSTER. This engine needs to send the range requests to all nodes, and
sending many of those requests at once reduces the communication costs significantly. This variable was added
in MySQL 5.0.3.
� named_pipe
(Windows only.) Indicates whether the server supports connections over named pipes.
� ndb_autoincrement_prefetch_sz
Determines the probability of gaps in an autoincremented column. Set to 1 to minimize this. Set to a high
value for optimization -- makes inserts faster, but decreases the likelihood that consecutive autoincrement
numbers will be used in a batch of inserts. Default value: 32. Mimimum value: 1.
� ndb_cache_check_time
The number of milliseconds to wait before checking the NDB query cache. Setting this to 0 (the default and
minimum value) means that the NDB query cache will be checked for validation on every query.
The recommended maximum value for this variable is 1000, which means that the query cache is checked once per
second. A larger value means the NDB query cache is less often checked and invalidated due to updates on a
different mysqld. It is generally not desirable to set this to a value greater than 2000.
� ndb_force_send
Forces sending of buffers to NDB immediately, without waiting for other threads. Defaults to ON.
� ndb_index_stat_cache_entries
Sets the granularity of the statistics by determining the number of starting and ending keys to store in the
statistics memory cache. Zero means no caching takes place; in this case, the data nodes are always queries
directly. Default value: 32.
� ndb_index_stat_enable
Use NDB index statistics in query optimization. Defaults to ON.
� ndb_index_stat_update_freq
How often to query data nodes instead of the statistics cache. For example, a value of 20 (the default) means
to direct every 20th query to the data nodes.
� ndb_report_thresh_binlog_epoch_slip
This is a threshold on the number of epochs to be behind before reporting binlog status. For example, a value
of 3 (the default) means that if the difference between which epoch has been received from the storage nodes
and which epoch has been applied to the binlog is 3 or more, a status message will be sent to the cluster log.
� ndb_report_thresh_binlog_mem_usage
This is a threshold on the percentage of free memory remaining before reporting binlog status. For example, a
value of 10 (the default) means that if the amount of available memory for receiving binlog data from the data
nodes falls below 10%, a status message will be sent to the cluster log.
� ndb_use_exact_count
Forces NDB to use an count of records during SELECT COUNT(*) query planning to speed up this type of query.
The default value is ON. For faster queries overall, disable this feature by setting the value of
ndb_use_exact_count to OFF.
� ndb_use_transactions
You can disable NDB transaction support by setting this variable's values to OFF (not recommended). The
default is ON.
� net_buffer_length
The communication buffer is reset to this size between SQL statements. 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 buffer is automatically enlarged, up to max_allowed_packet
bytes.
� 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 2.3, "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 5.3, "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 the maximum
number of tables used in a query plus 2, 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 pathname of the process ID (PID) file. This variable can be set with the --pid-file option.
� 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.
� protocol_version
The version of the client/server protocol used by the MySQL server.
� 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 a bit.
� 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 12.3, "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 12.3,
"Query Cache Configuration", for more information.
� 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: OptionDescription0 or OFFDon't cache results in or retrieve results from the
query cache. Note
that this does not deallocate the query cache
buffer. To do that, you should set
query_cache_size to 0.1 or ONCache all query results except for those that begin with
SELECT
SQL_NO_CACHE.2 or DEMANDCache results only for queries that begin with SELECT
SQL_CACHE.This variable defaults to ON.
� 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.
� read_only
When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except
from slave threads or from users that have the SUPER privilege. This can be useful to ensure that a slave
server 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.
� 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).
� 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. 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.
� rpl_recovery_rank
This variable is unused.
� 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 2.3, "Client does not support authentication protocol".
� server_id
The server ID. This value is set by the --server-id option. It is used for replication to enable master and
slave servers to identify themselves uniquely.
� 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.
� slave_compressed_protocol
Whether to use compression of the slave/master protocol if both the slave and the master support it.
� slave_load_tmpdir
The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements.
� slave_net_timeout
The number of seconds to wait for more data from a master/slave 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.
� slave_skip_errors
The replication errors that the slave should skip (ignore).
� slave_transaction_retries
If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or exceeded
InnoDB's innodb_lock_wait_timeout or NDBCluster's TransactionDeadlockDetectionTimeout or
TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an
error. The default priot to MySQL 4.0.3 is 0. You must explicitly set the value greater than 0 to enable the
"retry" behavior, which is probably a good idea. In MySQL 5.0.3 or newer, the default is 10.
� 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 4.4, "Where MySQL Stores Temporary Files".
� sql_mode
The current server SQL mode, which can be set dynamically. See the section called "THE SERVER SQL MODE".
� sql_slave_skip_counter
The number of events from the master that a slave server should skip. See Section 6.2.6, "SET GLOBAL
SQL_SLAVE_SKIP_COUNTER Syntax".
� 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. The cipher list has the same format as the openssl
ciphers command. 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 the section called "\FBMYSQLD\FR COMMAND OPTIONS".
� sync_binlog
If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using
fdatasync()) after every sync_binlog writes to the binary log. Note that there is one write to the binary log
per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which
does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at
most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk
has a battery-backed cache, which makes synchronization very fast).
If the value of sync_binlog is 0 (the default), no extra flushing is done. The server relies on the operating
system to flush the file contents occasionaly as for any other file.
� sync_frm
If this variable is set to 1, when any non-temporary 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 9.8, "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 the section called "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 4.8,
"How MySQL Opens and Closes Tables".
� table_lock_wait_timeout
Specifies a wait timeout for table-level locks, in seconds. The default timeout is 50 seconds. The timeout is
active only if the connection has open cursors. This variable can also be set globally at runtime (you need
the SUPER privilege to do this). It's available as of MySQL 5.0.10.
� 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 the section called "SERVER STATUS VARIABLES".
� thread_concurrency
On Solaris, mysqld calls thr_setconcurrency() with this 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.
The default is large enough for normal operation. See Section 1.4, "The MySQL Benchmark Suite". The default is
192KB.
� time_format
This variable is not implemented.
� time_zone
The current time zone. This variable is used to initialize the tome 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 9.8, "MySQL
Server Time Zone Support".
� tmp_table_size
The maximum size of in-memory temporary tables. (The actual limit is determined as the smaller of
max_heap_table_size and tmp_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.
� 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
filesystem 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.
However, if you are using MySQL 4.0.0 or later, 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 non-permanent 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 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';
� updatable_views_with_limit
This variable controls whether updates can be made using a view that does not contain a primary key in the
underlying table, if the update 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:
� 1 or YES: Issue a warning only (not an error message). This is the default value.
� 0 or NO: Prohibit the update.
This variable was added in MySQL 5.0.2.
� version
The version number for the server.
� 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.
� 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 non-interactive connection before closing it. This
timeout applies only to TCP/IP connections, not to connections made via Unix socket files, 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.
USING SYSTEM VARIABLES
The mysql server maintains many system variables that indicate how it is configured. the section called "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 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 gigabygtes, 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 the section called "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;
When you assign a value to a system variable with SET, you cannot use suffix letters in the value (as can be done
with startup options). However, the value can take the form of an expression:
SET sort_buffer_size = 10 * 1024 * 1024;
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 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-Max |
| version_comment | MySQL Community Edition - Max (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.