MySQL server maintains many system variables

The MySQL server maintains many system variables that configure its operation. 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 at runtime using the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can also use system variable values in expressions.

At runtime, setting a global system variable value requires the SUPER privilege. Setting a session system variable value normally requires no special privileges and can be done by any user, although there are exceptions. For more information, see Section 5.1.8.1, “System Variable Privileges”

There are several ways to see the names and values of system variables:

• To see the values that a server uses 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 uses based on only 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 or the Performance Schema system variable tables. See Section 25.12.13, “Performance Schema System Variable Tables”.

This section provides a description of each system variable. For a system variable summary table, see Section 5.1.4, “Server System Variable Reference”. For more information about manipulation of system variables, see Section 5.1.8, “Using System Variables”.

For additional system variable information, see these sections:

• Section 5.1.8, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
• Section 5.1.8.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
• Information on tuning system variables can be found in Section 5.1.1, “Configuring the Server”.
• Section 14.15, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.
• Section 21.4.3.9.2, “NDB Cluster System Variables”, lists system variables which are specific to NDB Cluster.
• For information on server system variables specific to replication, see Section 16.1.6, “Replication and Binary Logging Options and Variables”.

Note

Some of the following variable descriptions refer to “enabling” or “disabling” a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. Boolean variables can be set at startup to the values ON, TRUE, OFF, and FALSE (not case-sensitive), as well as 1 and 0. See Section 4.2.2.4, “Program Option Modifiers”.

Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible for the server to adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server sets the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Some system variables take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued variable is given as a relative path name, it is located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

authentication_windows_log_level

Command-Line Format	--authentication-windows-log-level=#
System Variable	authentication_windows_log_level
Scope	Global
Dynamic	No
Type	Integer
Default Value	2
Minimum Value	0
Maximum Value	4

This variable is available only if the authentication_windows Windows authentication plugin is enabled and debugging code is enabled. See Section 6.4.1.8, “Windows Pluggable Authentication”.

This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.

Value	Description
0	No logging
1	Log only error messages
2	Log level 1 messages and warning messages
3	Log level 2 messages and information notes
4	Log level 3 messages and debug messages

authentication_windows_use_principal_name

Command-Line Format	--authentication-windows-use-principal-name[={OFF|ON}]
System Variable	authentication_windows_use_principal_name
Scope	Global
Dynamic	No
Type	Boolean
Default Value	ON

• This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 6.4.1.8, “Windows Pluggable Authentication”.

A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName). MySQL uses the principal name (UPN) of the account under which the server is running. The UPN has the form user_id@computer_name and need not be registered anywhere to be used. This UPN is sent by the server at the beginning of authentication handshake.

This variable controls whether the server sends the UPN in the initial challenge. By default, the variable is enabled. For security reasons, it can be disabled to avoid sending the server’s account name to a client as cleartext. If the variable is disabled, the server always sends a 0x00 byte in the first challenge, the client does not specify targetName, and as a result, NTLM authentication is used.

If the server fails to obtain its UPN (which happens primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.

autocommit

Command-Line Format	--autocommit[={OFF|ON}]
System Variable	autocommit
Scope	Global, Session
Dynamic	Yes
Type	Boolean
Default Value	ON

The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Statements”.

By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the global autocommit value by starting the server with the --autocommit=0 option. To set the variable using an option file, include these lines:

[mysqld]
autocommit=0

automatic_sp_privileges

Command-Line Format	--automatic-sp-privileges[={OFF|ON}]
System Variable	automatic_sp_privileges
Scope	Global
Dynamic	Yes
Type	Boolean
Default Value	ON

When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

• If you start mysqld with --skip-new, automatic_sp_privileges is set to OFF.See also Section 23.2.2, “Stored Routines and MySQL Privileges”.
• auto_generate_certs

Command-Line Format	--auto-generate-certs[={OFF|ON}]
System Variable	auto_generate_certs
Scope	Global
Dynamic	No
Type	Boolean
Default Value	ON

• This variable is available if the server was compiled using OpenSSL (see Section 6.3.4, “SSL Library-Dependent Capabilities”). It controls whether the server autogenerates SSL key and certificate files in the data directory, if they do not already exist.

At startup, the server automatically generates server-side and client-side SSL certificate and key files in the data directory if the auto_generate_certs system variable is enabled, no SSL options other than --ssl are specified, and the server-side SSL files are missing from the data directory. These files enable secure client connections using SSL; see Section 6.3.1, “Configuring MySQL to Use Encrypted Connections”.

For more information about SSL file autogeneration, including file names and characteristics, see Section 6.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”

The sha256_password_auto_generate_rsa_keys system variable is related but controls autogeneration of RSA key-pair files needed for secure password exchange using RSA over unencypted connections.

avoid_temporal_upgrade

Command-Line Format	--avoid-temporal-upgrade[={OFF|ON}]
Deprecated	Yes
System Variable	avoid_temporal_upgrade
Scope	Global
Dynamic	Yes
Type	Boolean
Default Value	OFF

• This variable controls whether ALTER TABLE implicitly upgrades temporal columns found to be in pre-5.6.4 format (TIME, DATETIME, and TIMESTAMP columns without support for fractional seconds precision). Upgrading such columns requires a table rebuild, which prevents any use of fast alterations that might otherwise apply to the operation to be performed.

This variable is disabled by default. Enabling it causes ALTER TABLE not to rebuild temporal columns and thereby be able to take advantage of possible fast alterations.

This variable is deprecated; expect it to be removed in a future release of MySQL.

back_log

Command-Line Format	--back-log=#
System Variable	back_log
Scope	Global
Dynamic	No
Type	Integer
Default Value	-1 (signifies autosizing; do not assign this literal value)
Minimum Value	1
Maximum Value	65535

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.

The default value is based on the following formula, capped to a limit of 900:

• 50 + (max_connections / 5)

basedir

Command-Line Format	--basedir=dir_name
System Variable	basedir
Scope	Global
Dynamic	No
Type	Directory name
Default Value	configuration-dependent default

• The path to the MySQL installation base directory.

big_tables

Command-Line Format	--big-tables[={OFF|ON}]
System Variable	big_tables
Scope	Global, Session
Dynamic	Yes
Type	Boolean
Default Value	OFF

If enabled, the server stores all temporary tables on disk rather than in memory. This prevents most The table tbl_name is full errors for SELECT operations that require a large temporary table, but also slows down queries for which in-memory tables would suffice.

The default value for new connections is OFF (use in-memory temporary tables). Normally, it should never be necessary to enable this variable because the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk-based tables as required.

bind_address

Command-Line Format	--bind-address=addr
System Variable	bind_address
Scope	Global
Dynamic	No
Type	String
Default Value	*

The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. To specify an address, set bind_address=addr at server startup, where addr is an IPv4 or IPv6 address or a host name. If addr is a host name, the server resolves the name to an IP address and binds to that address. If a host name resolves to multiple IP addresses, the server uses the first IPv4 address if there are any, or the first IPv6 address otherwise.

The server treats different types of addresses as follows:

• If the address is *, the server accepts TCP/IP connections on all server host IPv4 interfaces, and, if the server host supports IPv6, on all IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces. This value is the default.
• If the address is 0.0.0.0, the server accepts TCP/IP connections on all server host IPv4 interfaces.
• If the address is ::, the server accepts TCP/IP connections on all server host IPv4 and IPv6 interfaces.
• If the address is an IPv4-mapped address, the server accepts TCP/IP connections for that address, in either IPv4 or IPv6 format. For example, if the server is bound to ::ffff:127.0.0.1, clients can connect using --host=127.0.0.1 or --host=::ffff:127.0.0.1.
• If the address is a “regular” IPv4 or IPv6 address (such as 127.0.0.1 or ::1), the server accepts TCP/IP connections only for that IPv4 or IPv6 address.

If binding to the address fails, the server produces an error and does not start.

If you intend to bind the server to a specific address, be sure that the mysql.user system table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you cannot shut down the server. For example, if you bind the server to *, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

This variable has no effect for the embedded server (libmysqld) and is not visible within the embedded server.

block_encryption_mode

Command-Line Format	--block-encryption-mode=#
System Variable	block_encryption_mode
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	aes-128-ecb

This variable controls the block encryption mode for block-based algorithms such as AES. It affects encryption for AES_ENCRYPT() and AES_DECRYPT().

block_encryption_mode takes a value in aes-keylen-mode format, where keylen is the key length in bits and mode is the encryption mode. The value is not case-sensitive. Permitted keylen values are 128, 192, and 256. Permitted encryption modes depend on whether MySQL was compiled using OpenSSL or yaSSL:

• For OpenSSL, permitted mode values are: ECB, CBC, CFB1, CFB8, CFB128, OFB
• For yaSSL, permitted mode values are: ECB, CBC

For example, this statement causes the AES encryption functions to use a key length of 256 bits and the CBC mode:

SET block_encryption_mode = 'aes-256-cbc';

An error occurs for attempts to set block_encryption_mode to a value containing an unsupported key length or a mode that the SSL library does not support.

bulk_insert_buffer_size

Command-Line Format	--bulk-insert-buffer-size=#
System Variable	bulk_insert_buffer_size
Scope	Global, Session
Dynamic	Yes
Type	Integer
Default Value	8388608
Minimum Value	0
Maximum Value (64-bit platforms)	18446744073709551615
Maximum Value (32-bit platforms)	4294967295
Unit	bytes/thread

MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA when adding data to nonempty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

character_set_client

System Variable	character_set_client
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	utf8

• The character set for statements that arrive from the client. The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 10.4, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:
  • The client requests a character set not known to the server. For example, a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.
  • The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.
  • mysqld was started with the --skip-character-set-client-handshake option, which causes it to ignore client character set configuration. This reproduces MySQL 4.0 behavior and is useful should you wish to upgrade the server without upgrading all the clients.
  Some character sets cannot be used as the client character set. Attempting to use them as the character_set_client value produces an error. See Impermissible Client Character Sets.
• character_set_connection

System Variable	character_set_connection
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	utf8

The character set used for literals specified without a character set introducer and for number-to-string conversion. For information about introducers, see Section 10.3.8, “Character Set Introducers”.

character_set_database

System Variable	character_set_database
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	latin1
Footnote	This option is dynamic, but should be set only by server. You should not set this variable manually.

• 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.

The global character_set_database and collation_database system variables are deprecated in MySQL 5.7; expect them to be removed in a future version of MySQL.

Assigning a value to the session character_set_database and collation_database system variables is deprecated in MySQL 5.7 and assignments produce a warning. You should expect the session variables to become read only in a future version of MySQL and assignments to produce an error, while remaining possible to access the session variables to determine the database character set and collation for the default database.

character_set_filesystem

Command-Line Format	--character-set-filesystem=name
System Variable	character_set_filesystem
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	binary

The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multibyte file names are permitted, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8mb4'.

character_set_results

System Variable	character_set_results
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	utf8

The character set used for returning query results to the client. This includes result data such as column values, result metadata such as column names, and error messages.

character_set_server

Command-Line Format	--character-set-server=name
System Variable	character_set_server
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	latin1

The servers default character set. See Section 10.15, “Character Set Configuration”. If you set this variable, you should also set collation_server to specify the collation for the character set.

character_set_system

System Variable	character_set_system
Scope	Global
Dynamic	No
Type	String
Default Value	utf8

The character set used by the server for storing identifiers. The value is always utf8.

character_sets_dir

Command-Line Format	--character-sets-dir=dir_name
System Variable	character_sets_dir
Scope	Global
Dynamic	No
Type	Directory name

The directory where character sets are installed. See Section 10.15, “Character Set Configuration”.

check_proxy_users

Command-Line Format	--check-proxy-users[={OFF|ON}]
System Variable	check_proxy_users
Scope	Global
Dynamic	Yes
Type	Boolean
Default Value	OFF

• Some authentication plugins implement proxy user mapping for themselves (for example, the PAM and Windows authentication plugins). Other authentication plugins do not support proxy users by default. Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password, sha256_password.

If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request. However, it may also be necessary to enable plugin-specific system variables to take advantage of server proxy user mapping support:

• For the mysql_native_password plugin, enable mysql_native_password_proxy_users.

For the sha256_password plugin, enable sha256_password_proxy_users.

For information about user proxying, see Section 6.2.14, “Proxy Users”.

collation_connection

System Variable	collation_connection
Scope	Global, Session
Dynamic	Yes
Type	String

The collation of the connection character set. collation_connection is important for comparisons of literal strings. For comparisons of strings with column values, collation_connection does not matter because columns have their own collation, which has a higher collation precedence (see Section 10.8.4, “Collation Coercibility in Expressions”).

collation_database

System Variable	collation_database
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	latin1_swedish_ci
Footnote	This option is dynamic, but should be set only by server. You should not set this variable manually.

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.

The global character_set_database and collation_database system variables are deprecated in MySQL 5.7; expect them to be removed in a future version of MySQL.

Assigning a value to the session character_set_database and collation_database system variables is deprecated in MySQL 5.7 and assignments produce a warning. Expect the session variables to become read only in a future version of MySQL and assignments to produce an error, while remaining possible to access the session variables to determine the database character set and collation for the default database

collation_server

Command-Line Format	--collation-server=name
System Variable	collation_server
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value	latin1_swedish_ci

The server’s default collation. See Section 10.15, “Character Set Configuration”.

completion_type

Command-Line Format	--completion-type=#
System Variable	completion_type
Scope	Global, Session
Dynamic	Yes
Type	Enumeration
Default Value	NO_CHAIN
Valid Values	NO_CHAINCHAINRELEASE012

The transaction completion type. This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

Value	Description
NO_CHAIN (or 0)	COMMIT and ROLLBACK are unaffected. This is the default value.
CHAIN (or 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.)
RELEASE (or 2)	COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 13.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

concurrent_insert

Command-Line Format	--concurrent-insert[=value]
System Variable	concurrent_insert
Scope	Global
Dynamic	Yes
Type	Enumeration
Default Value	AUTO
Valid Values	NEVERAUTOALWAYS012

If AUTO (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file.

This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

Value	Description
NEVER (or 0)	Disables concurrent inserts
AUTO (or 1)	(Default) Enables concurrent insert for MyISAM tables that do not have holes
ALWAYS (or 2)	Enables concurrent inserts for all MyISAM tables, even those that have holes. For a table with a hole, new rows are inserted at the end of the table if it is in use by another thread. Otherwise, MySQL acquires a normal write lock and inserts the row into the hole.

If you start mysqld with --skip-new, concurrent_insert is set to NEVER.

See also Section 8.11.3, “Concurrent Inserts”.

connect_timeout

Command-Line Format	--connect-timeout=#
System Variable	connect_timeout
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	10
Minimum Value	2
Maximum Value	31536000
Unit	seconds

The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds.

Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.

core_file

System Variable	core_file
Scope	Global
Dynamic	No
Type	Boolean
Default Value	OFF

Whether to write a core file if the server unexpectedly exits. This variable is set by the --core-file option.

datadir

Command-Line Format	--datadir=dir_name
System Variable	datadir
Scope	Global
Dynamic	No
Type	Directory name

• The path to the MySQL server data directory. Relative paths are resolved with respect to the current directory. If you expect the server to be started automatically (that is, in contexts for which you cannot assume what the current directory is), it is best to specify the datadir value as an absolute path.
• date_format
• This variable is unused. It is deprecated and is removed in MySQL 8.0.
• datetime_format
• This variable is unused. It is deprecated and is removed in MySQL 8.0.

debug

Command-Line Format	--debug[=debug_options]
System Variable	debug
Scope	Global, Session
Dynamic	Yes
Type	String
Default Value (Unix)	d:t:i:o,/tmp/mysqld.trace
Default Value (Windows)	d:t:i:O,\mysqld.trace

This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.8.1, “System Variable Privileges”.

Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:

mysql> SET debug = 'T';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| T       |
+---------+

mysql> SET debug = '+P';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| P:T     |
+---------+

mysql> SET debug = '-P';
mysql> SELECT @@debug;
+---------+
| @@debug |
+---------+
| T       |
+---------+

For more information, see Section 5.8.3, “The DBUG Package”.

debug_sync

System Variable	debug_sync
Scope	Session
Dynamic	Yes
Type	String

• This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.7, “MySQL Source-Configuration Options”). If Debug Sync is not compiled in, this system variable is not available.

The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.

The session value can be read by any user and has the same value as the global variable. The session value can be set to control synchronization points.

Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.8.1, “System Variable Privileges”.

For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

default_authentication_plugin

Command-Line Format	--default-authentication-plugin=plugin_name
System Variable	default_authentication_plugin
Scope	Global
Dynamic	No
Type	Enumeration
Default Value	mysql_native_password
Valid Values	mysql_native_passwordsha256_password

The default authentication plugin. These values are permitted:

• mysql_native_password: Use MySQL native passwords; see Section 6.4.1.1, “Native Pluggable Authentication”.
• sha256_password: Use SHA-256 passwords; see Section 6.4.1.5, “SHA-256 Pluggable Authentication”.

Note

If this variable has a value other than mysql_native_password, clients older than MySQL 5.5.7 cannot connect because, of the permitted default authentication plugins, they understand only the mysql_native_password authentication protocol.

The default_authentication_plugin value affects these aspects of server operation:

• It determines which authentication plugin the server assigns to new accounts created by CREATE USER and GRANT statements that do not explicitly specify an authentication plugin.

• The old_passwords system variable affects password hashing for accounts that use the mysql_native_password or sha256_password authentication plugin. If the default authentication plugin is one of those plugins, the server sets old_passwords at startup to the value required by the plugin password hashing method.
• For an account created with either of the following statements, the server associates the account with the default authentication plugin and assigns the account the given password, hashed as required by that plugin:
• CREATE USER … IDENTIFIED BY ‘cleartext password‘;
• GRANT … IDENTIFIED BY ‘cleartext password‘;

For an account created with either of the following statements, the server associates the account with the default authentication plugin and assigns the account the given password hash, if the password hash has the format required by the plugin:

CREATE USER ... IDENTIFIED BY PASSWORD 'encrypted password';
GRANT ...  IDENTIFIED BY PASSWORD 'encrypted password';
If the password hash is not in the format required by the default authentication plugin, the statement fails.
default_password_lifetime
Command-Line Format	--default-password-lifetime=#
System Variable	default_password_lifetime
Scope	Global
Dynamic	Yes
Type	Integer
Default Value (≥ 5.7.11)	0
Default Value (≤ 5.7.10)	360
Minimum Value	0
Maximum Value	65535
Unit	days
This variable defines the global automatic password expiration policy. The default default_password_lifetime value is 0, which disables automatic password expiration. If the value of default_password_lifetime is a positive integer N, it indicates the permitted password lifetime; passwords must be changed every N days.
The global password expiration policy can be overridden as desired for individual accounts using the password expiration options of the ALTER USER statement. See Section 6.2.11, “Password Management”.
Note
Prior to MySQL 5.7.11, the default default_password_lifetime value is 360 (passwords must be changed approximately once per year). For those versions, be aware that, if you make no changes to the default_password_lifetime variable or to individual user accounts, all user passwords expire after 360 days, and all user accounts start running in restricted mode when this happens. Clients (which are effectively users) connecting to the server then get an error indicating that the password must be changed: ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.
However, this is easy to miss for clients that automatically connect to the server, such as connections made from scripts. To avoid having such clients suddenly stop working due to a password expiring, make sure to change the password expiration settings for those clients, like this:
ALTER USER 'script'@'localhost' PASSWORD EXPIRE NEVER;
Alternatively, set the default_password_lifetime variable to 0, thus disabling automatic password expiration for all users.
default_storage_engine
Command-Line Format	--default-storage-engine=name
System Variable	default_storage_engine
Scope	Global, Session
Dynamic	Yes
Type	Enumeration
Default Value	InnoDB
The default storage engine for tables. See Chapter 15, Alternative Storage Engines. This variable sets the storage engine for permanent tables only. To set the storage engine for TEMPORARY tables, set the default_tmp_storage_engine system variable.
To see which storage engines are available and enabled, use the SHOW ENGINES statement or query the INFORMATION_SCHEMA ENGINES table.
If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine or the server cannot start.
default_tmp_storage_engine
Command-Line Format	--default-tmp-storage-engine=name
System Variable	default_tmp_storage_engine
Scope	Global, Session
Dynamic	Yes
Type	Enumeration
Default Value	InnoDB
The default storage engine for TEMPORARY tables (created with CREATE TEMPORARY TABLE). To set the storage engine for permanent tables, set the default_storage_engine system variable. Also see the discussion of that variable regarding possible values.
If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine or the server cannot start.
default_week_format
Command-Line Format	--default-week-format=#
System Variable	default_week_format
Scope	Global, Session
Dynamic	Yes
Type	Integer
Default Value	0
Minimum Value	0
Maximum Value	7
The default mode value to use for the WEEK() function. See Section 12.7, “Date and Time Functions”.
delay_key_write
Command-Line Format	--delay-key-write[={OFF|ON|ALL}]
System Variable	delay_key_write
Scope	Global
Dynamic	Yes
Type	Enumeration
Default Value	ON
Valid Values	
OFF
ON
ALL
This variable specifies how to use delayed key writes. It applies only to MyISAM tables. Delayed key writing causes key buffers not to be flushed between writes. See also Section 15.2.1, “MyISAM Startup Options”.
This variable 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.
Option	Description
OFF	DELAY_KEY_WRITE is ignored.
ON	MySQL honors any DELAY_KEY_WRITE option specified in CREATE TABLE statements. This is the default value.
ALL	All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.
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.
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_options system variable set (for example, myisam_recover_options='BACKUP,FORCE'). See Section 5.1.7, “Server System Variables”, and Section 15.2.1, “MyISAM Startup Options”.
If you start mysqld with --skip-new, delay_key_write is set to OFF.
Warning
If you enable external locking with --external-locking, there is no protection against index corruption for tables that use delayed key writes.
delayed_insert_limit
Command-Line Format	--delayed-insert-limit=#
Deprecated	Yes
System Variable	delayed_insert_limit
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	100
Minimum Value	1
Maximum Value (64-bit platforms)	18446744073709551615
Maximum Value (32-bit platforms)	4294967295
This system variable is deprecated (because DELAYED inserts are not supported); expect it to be removed in a future release.
delayed_insert_timeout
Command-Line Format	--delayed-insert-timeout=#
Deprecated	Yes
System Variable	delayed_insert_timeout
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	300
Minimum Value	1
Maximum Value	31536000
Unit	seconds
This system variable is deprecated (because DELAYED inserts are not supported); expect it to be removed in a future release.
delayed_queue_size
Command-Line Format	--delayed-queue-size=#
Deprecated	Yes
System Variable	delayed_queue_size
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	1000
Minimum Value	1
Maximum Value (64-bit platforms)	18446744073709551615
Maximum Value (32-bit platforms)	4294967295
This system variable is deprecated (because DELAYED inserts are not supported); expect it to be removed in a future release.
disabled_storage_engines
Command-Line Format	--disabled-storage-engines=engine[,engine]...
System Variable	disabled_storage_engines
Scope	Global
Dynamic	No
Type	String
Default Value	empty string
This variable indicates which storage engines cannot be used to create tables or tablespaces. For example, to prevent new MyISAM or FEDERATED tables from being created, start the server with these lines in the server option file:
[mysqld]
disabled_storage_engines="MyISAM,FEDERATED"
By default, disabled_storage_engines is empty (no engines disabled), but it can be set to a comma-separated list of one or more engines (not case-sensitive). Any engine named in the value cannot be used to create tables or tablespaces with CREATE TABLE or CREATE TABLESPACE, and cannot be used with ALTER TABLE ... ENGINE or ALTER TABLESPACE ... ENGINE to change the storage engine of existing tables or tablespaces. Attempts to do so result in an ER_DISABLED_STORAGE_ENGINE error.
disabled_storage_engines does not restrict other DDL statements for existing tables, such as CREATE INDEX, TRUNCATE TABLE, ANALYZE TABLE, DROP TABLE, or DROP TABLESPACE. This permits a smooth transition so that existing tables or tablespaces that use a disabled engine can be migrated to a permitted engine by means such as ALTER TABLE ... ENGINE permitted_engine.
It is permitted to set the default_storage_engine or default_tmp_storage_engine system variable to a storage engine that is disabled. This could cause applications to behave erratically or fail, although that might be a useful technique in a development environment for identifying applications that use disabled engines, so that they can be modified.
disabled_storage_engines is disabled and has no effect if the server is started with any of these options: --bootstrap, --initialize, --initialize-insecure, --skip-grant-tables.
Note
Setting disabled_storage_engines might cause an issue with mysql_upgrade. For details, see Section 4.4.7, “mysql_upgrade — Check and Upgrade MySQL Tables”.
disconnect_on_expired_password
Command-Line Format	--disconnect-on-expired-password[={OFF|ON}]
System Variable	disconnect_on_expired_password
Scope	Global
Dynamic	No
Type	Boolean
Default Value	ON
This variable controls how the server handles clients with expired passwords:
If the client indicates that it can handle expired passwords, the value of disconnect_on_expired_password is irrelevant. The server permits the client to connect but puts it in sandbox mode.
If the client does not indicate that it can handle expired passwords, the server handles the client according to the value of disconnect_on_expired_password:
If disconnect_on_expired_password: is enabled, the server disconnects the client.
If disconnect_on_expired_password: is disabled, the server permits the client to connect but puts it in sandbox mode.
For more information about the interaction of client and server settings relating to expired-password handling, see Section 6.2.12, “Server Handling of Expired Passwords”.
div_precision_increment
Command-Line Format	--div-precision-increment=#
System Variable	div_precision_increment
Scope	Global, Session
Dynamic	Yes
Type	Integer
Default Value	4
Minimum Value	0
Maximum Value	30
This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.
mysql> SELECT 1/7;
+--------+
| 1/7    |
+--------+
| 0.1429 |
+--------+
mysql> SET div_precision_increment = 12;
mysql> SELECT 1/7;
+----------------+
| 1/7            |
+----------------+
| 0.142857142857 |
+----------------+
end_markers_in_json
Command-Line Format	--end-markers-in-json[={OFF|ON}]
System Variable	end_markers_in_json
Scope	Global, Session
Dynamic	Yes
Type	Boolean
Default Value	OFF
Whether optimizer JSON output should add end markers. See MySQL Internals: The end_markers_in_json System Variable.
eq_range_index_dive_limit
Command-Line Format	--eq-range-index-dive-limit=#
System Variable	eq_range_index_dive_limit
Scope	Global, Session
Dynamic	Yes
Type	Integer
Default Value	200
Minimum Value	0
Maximum Value	4294967295
This variable indicates the number of equality ranges in an equality comparison condition when the optimizer should switch from using index dives to index statistics in estimating the number of qualifying rows. It applies to evaluation of expressions that have either of these equivalent forms, where the optimizer uses a nonunique index to look up col_name values:
col_name IN(val1, ..., valN)
col_name = val1 OR ... OR col_name = valN
In both cases, the expression contains N equality ranges. The optimizer can make row estimates using index dives or index statistics. If eq_range_index_dive_limit is greater than 0, the optimizer uses existing index statistics instead of index dives if there are eq_range_index_dive_limit or more equality ranges. Thus, to permit use of index dives for up to N equality ranges, set eq_range_index_dive_limit to N + 1. To disable use of index statistics and always use index dives regardless of N, set eq_range_index_dive_limit to 0.
For more information, see Equality Range Optimization of Many-Valued Comparisons.
To update table index statistics for best estimates, use ANALYZE TABLE.
error_count
The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.17, “SHOW ERRORS Statement”.
event_scheduler
Command-Line Format	--event-scheduler[=value]
System Variable	event_scheduler
Scope	Global
Dynamic	Yes
Type	Enumeration
Default Value	OFF
Valid Values	
OFF
ON
DISABLED
This variable enables or disables, and starts or stops, the Event Scheduler. The possible status values are ON, OFF, and DISABLED. Turning the Event Scheduler OFF is not the same as disabling the Event Scheduler, which requires setting the status to DISABLED. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in Section 23.4.2, “Event Scheduler Configuration”
explicit_defaults_for_timestamp
Command-Line Format	--explicit-defaults-for-timestamp[={OFF|ON}]
Deprecated	Yes
System Variable	explicit_defaults_for_timestamp
Scope	Global, Session
Dynamic	Yes
Type	Boolean
Default Value	OFF
This system variable determines whether the server enables certain nonstandard behaviors for default values and NULL-value handling in TIMESTAMP columns. By default, explicit_defaults_for_timestamp is disabled, which enables the nonstandard behaviors.
If explicit_defaults_for_timestamp is disabled, the server enables the nonstandard behaviors and handles TIMESTAMP columns as follows:
TIMESTAMP columns not explicitly declared with the NULL attribute are automatically declared with the NOT NULL attribute. Assigning such a column a value of NULL is permitted and sets the column to the current timestamp.
The first TIMESTAMP column in a table, if not explicitly declared with the NULL attribute or an explicit DEFAULT or ON UPDATE attribute, is automatically declared with the DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP attributes.
TIMESTAMP columns following the first one, if not explicitly declared with the NULL attribute or an explicit DEFAULT attribute, are automatically declared as DEFAULT '0000-00-00 00:00:00' (the “zero” timestamp). For inserted rows that specify no explicit value for such a column, the column is assigned '0000-00-00 00:00:00' and no warning occurs.
Depending on whether strict SQL mode or the NO_ZERO_DATE SQL mode is enabled, a default value of '0000-00-00 00:00:00' may be invalid. Be aware that the TRADITIONAL SQL mode includes strict mode and NO_ZERO_DATE. See Section 5.1.10, “Server SQL Modes”.
The nonstandard behaviors just described are deprecated; expect them to be removed in a future release of MySQL.
If explicit_defaults_for_timestamp is enabled, the server disables the nonstandard behaviors and handles TIMESTAMP columns as follows:
It is not possible to assign a TIMESTAMP column a value of NULL to set it to the current timestamp. To assign the current timestamp, set the column to CURRENT_TIMESTAMP or a synonym such as NOW().
TIMESTAMP columns not explicitly declared with the NOT NULL attribute are automatically declared with the NULL attribute and permit NULL values. Assigning such a column a value of NULL sets it to NULL, not the current timestamp.
TIMESTAMP columns declared with the NOT NULL attribute do not permit NULL values. For inserts that specify NULL for such a column, the result is either an error for a single-row insert if strict SQL mode is enabled, or '0000-00-00 00:00:00' is inserted for multiple-row inserts with strict SQL mode disabled. In no case does assigning the column a value of NULL set it to the current timestamp.
TIMESTAMP columns explicitly declared with the NOT NULL attribute and without an explicit DEFAULT attribute are treated as having no default value. For inserted rows that specify no explicit value for such a column, the result depends on the SQL mode. If strict SQL mode is enabled, an error occurs. If strict SQL mode is not enabled, the column is declared with the implicit default of '0000-00-00 00:00:00' and a warning occurs. This is similar to how MySQL treats other temporal types such as DATETIME.
No TIMESTAMP column is automatically declared with the DEFAULT CURRENT_TIMESTAMP or ON UPDATE CURRENT_TIMESTAMP attributes. Those attributes must be explicitly specified.
The first TIMESTAMP column in a table is not handled differently from TIMESTAMP columns following the first one.
If explicit_defaults_for_timestamp is disabled at server startup, this warning appears in the error log:
[Warning] TIMESTAMP with implicit DEFAULT value is deprecated.
Please use --explicit_defaults_for_timestamp server option (see
documentation for more details).
As indicated by the warning, to disable the deprecated nonstandard behaviors, enable the explicit_defaults_for_timestamp system variable at server startup.
Note
explicit_defaults_for_timestamp is itself deprecated because its only purpose is to permit control over deprecated TIMESTAMP behaviors that are to be removed in a future release of MySQL. When removal of those behaviors occurs, explicit_defaults_for_timestamp no longer has any purpose, and you can expect it to be removed as well.
For additional information, see Section 11.2.6, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.
external_user
System Variable	external_user
Scope	Session
Dynamic	No
Type	String
The external user name used during the authentication process, as set by the plugin used to authenticate the client. With native (built-in) MySQL authentication, or if the plugin does not set the value, this variable is NULL. See Section 6.2.14, “Proxy Users”.
flush
Command-Line Format	--flush[={OFF|ON}]
System Variable	flush
Scope	Global
Dynamic	Yes
Type	Boolean
Default Value	OFF
If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.
Note
If flush is enabled, the value of flush_time does not matter and changes to flush_time have no effect on flush behavior.
flush_time
Command-Line Format	--flush-time=#
System Variable	flush_time
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	0
Minimum Value	0
Maximum Value	31536000
Unit	seconds
If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources.
Note
If flush is enabled, the value of flush_time does not matter and changes to flush_time have no effect on flush behavior.
foreign_key_checks
System Variable	foreign_key_checks
Scope	Global, Session
Dynamic	Yes
Type	Boolean
Default Value	ON
If set to 1 (the default), foreign key constraints are checked. If set to 0, foreign key constraints are ignored, with a couple of exceptions. When re-creating a table that was dropped, an error is returned if the table definition does not conform to the foreign key constraints referencing the table. Likewise, an ALTER TABLE operation returns an error if a foreign key definition is incorrectly formed. For more information, see Section 13.1.18.5, “FOREIGN KEY Constraints”.
Setting this variable has the same effect on NDB tables as it does for InnoDB tables. Typically you leave this setting enabled during normal operation, to enforce referential integrity. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 13.1.18.5, “FOREIGN KEY Constraints”.
Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.
Note
Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks=0 are not verified for consistency.
Dropping an index required by a foreign key constraint is not permitted, even with foreign_key_checks=0. The foreign key constraint must be removed before dropping the index (Bug #70260).
source : https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_execution_time