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19.2 Event Scheduler Syntax »
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19.1. Event Scheduler Overview

MySQL Events are tasks that run according to a schedule. Therefore, we sometimes refer to them as scheduled events. When you create an event, you are creating a named database object containing one or more SQL statements to be executed at one or more regular intervals, beginning and ending at a specific date and time. Conceptually, this is similar to the idea of the Unix crontab (also known as a 「cron job」) or the Windows Task Scheduler.

Scheduled tasks of this type are also sometimes known as 「temporal triggers」, implying that these are objects that are triggered by the passage of time. While this is essentially correct, we prefer to use the term events in order to avoid confusion with triggers of the type discussed in 章 18. トリガ. Events should more specifically not be confused with 「temporary triggers」. Whereas a trigger is a database object whose statements are executed in response to a specific type of event that occurs on a given table, a (scheduled) event is an object whose statements are executed in response to the passage of a specified time interval.

While there is no provision in the SQL Standard for event scheduling, there are precedents in other database systems, and you may notice some similarities between these implementations and that found in the MySQL Server.

MySQL Events have the following major features and properties:

  • In MySQL 5.1.12 and later, an event is uniquely identified by its name and the schema to which it is assigned. (Previously, an event was also unique to its definer.)

  • An event performs a specific action according to a schedule. This action consists of an SQL statement, which can be a compound statement in a BEGIN ... END block if desired (see 項17.2.5. 「BEGIN ... END 複合ステートメント構文」). An event's timing can be either one-time or recurrent. A one-time event executes one time only. A recurrent event repeats its action at a regular interval, and the schedule for a recurring event can be assigned a specific start day and time, end day and time, both, or neither. (By default, a recurring event's schedule begins as soon as it is created, and continues indefinitely, until it is disabled or dropped.)

  • Users can create, modify, and drop scheduled events using SQL statements intended for these purposes. Syntactically invalid event creation and modification statements fail with an appropriate error message. A user may include statements in an event's action which require privileges that the user does not actually have. The event creation or modification statement succeeds but the event's action fails. See 項19.5. 「The Event Scheduler and MySQL Privileges」 for details.

  • Many of the properties of an event can be set or modified using SQL statements. These properties include the event's name, timing, persistence (that is, whether it is preserved following the expiration of its schedule), status (enabled or disabled), action to be performed, and the schema to which it is assigned. See 項19.2.2. 「ALTER EVENT Syntax」.

    The definer of an event is the user who created the event, unless the event has been altered, in which case the definer is the user who issued the last ALTER EVENT statement effecting that event. An event can be modified by any user having the EVENT privilege on the database for which the event is defined. (Prior to MySQL 5.1.12, only an event's definer, or a user having privileges on the mysql.event table, could modify a given event.) See 項19.5. 「The Event Scheduler and MySQL Privileges」.

  • An event's action statement may include most SQL statements permitted within stored routines.

Events are executed by a special event scheduler thread; when we refer to the Event Scheduler, we actually refer to this thread. When running, the event scheduler thread and its current state can be seen by users having the SUPER privilege in the output of SHOW PROCESSLIST, as shown in the discussion that follows.

The global variable event_scheduler determines whether the Event Scheduler is enabled and running on the server. Beginning with MySQL 5.1.12, it has one of these 3 values, which affect event scheduling as described here:

  • OFF: The Event Scheduler is stopped. The event scheduler thread does not run, is not shown in the output of SHOW PROCESSLIST, and no scheduled events are executed. OFF is the default value for event_scheduler.

    When the Event Scheduler is stopped (event_scheduler is OFF), it can be started by setting the value of event_scheduler to ON. (See next item.)

  • ON: The Event Scheduler is started; the event scheduler thread runs and executes all scheduled events.

    When the Event Scheduler is ON, the event scheduler thread is listed in the output of SHOW PROCESSLIST as a daemon process, and its state is represented as shown here: 

    mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 1
   User: root
   Host: localhost
     db: NULL
Command: Query
   Time: 0
  State: NULL
   Info: show processlist
*************************** 2. row ***************************
     Id: 2
   User: event_scheduler
   Host: localhost
     db: NULL
Command: Daemon
   Time: 3
  State: Waiting for next activation
   Info: NULL
2 rows in set (0.00 sec)

    Event scheduling can be stopped by setting the value of event_scheduler to OFF.

  • DISABLED: This value renders the Event Scheduler non-operational. When the Event Scheduler is DISABLED, the event scheduler thread does not run (and so does not appear in the output of SHOW PROCESSLIST).

When the server is runningevent_scheduler can be toggled between ON and OFF (using SET). It is also possible to use 0 for OFF, and 1 for ON when setting this variable. Thus, any of the following 4 statements can be used in the mysql client to turn on the Event Scheduler: 

SET GLOBAL event_scheduler = ON;
SET @@global.event_scheduler = ON;
SET GLOBAL event_scheduler = 1;
SET @@global.event_scheduler = 1;

Similarly, any of these 4 statements can be used to turn off the Event Scheduler: 

SET GLOBAL event_scheduler = OFF;
SET @@global.event_scheduler = OFF;
SET GLOBAL event_scheduler = 0;
SET @@global.event_scheduler = 0;

Although ON and OFF have numeric equivalents, the value displayed for event_scheduler by SELECT or SHOW VARIABLES is always one of OFF, ON, or DISABLED. DISABLED has no numeric equivalent. For this reason, ON and OFF are usually preferred over 1 and 0 when setting this variable.

Note that attempting to set event_scheduler without specifying it as a global variable causes an error: 

mysql< SET @@event_scheduler = OFF;
ERROR 1229 (HY000): Variable 'event_scheduler' is a GLOBAL
variable and should be set with SET GLOBAL

Important: It is not possible to enable or disable the Event Scheduler when the server is running. That is, you can change the value of event_scheduler to DISABLED — or from DISABLED to one of the other permitted values for this option — only when the server is stopped. Attempting to do so when the server is running fails with an error.

To disable the event scheduler, use one of the following two methods:

  • As a command-line option when starting the server: 

    --event-scheduler=DISABLED

  • In the server configuration file (my.cnf, or my.ini on Windows systems), include the line where it will be read by the server (for example, in a [mysqld] section): 

    event_scheduler=DISABLED

To enable the Event Scheduler, restart the server without the --event-scheduler=DISABLED command line option, or after removing or commenting out the line containing event_scheduler=DISABLED in the server configuration file, as appropriate. Alternatively, you can use ON (or 1) or OFF (or 0) in place of the DISABLED value when starting the server.

Note: You can issue event-manipulation statements when event_scheduler is set to DISABLED. No warnings or errors are generated in such cases (provided that the statements are themselves valid). However, scheduled events cannot execute until this variable is set to ON (or 1). Once this has been done, the event scheduler thread executes all events whose scheduling conditions are satisfied.

In MySQL 5.1.11, event_scheduler behaved as follows: this variable could take one of the values 0 (or OFF), 1 (or ON), or 2. Setting it to 0 turned event scheduling off, so that the event scheduler thread did not run; the event_scheduler variable could not be set to this value while the server was running. Setting it to 1 so that the event scheduler thread ran and executed scheduled events. In this state, the event scheduler thread appeared to be sleeping when viewed with SHOW PROCESSLIST. When event_scheduler was set to 2 (which was the default value), the Event Scheduler was considered to be 「suspended」; the event scheduler thread ran and could be seen in the output of SHOW PROCESSLIST (where Suspended was displayed in the State column), but did not execute any scheduled events. The value of event_scheduler could be changed only between 1 (or ON) and 2 while the server was running. Setting it to 0 (or OFF) required a server restart, as did changing its value from 0 (or OFF) to 1 (or ON) or 2.

Prior to MySQL 5.1.11, event_scheduler could take one of only the 2 values 0|OFF or 1|ON, and the default value was 0|OFF. It was also possible to start and stop the event scheduler thread while the MySQL server was running.

For more information concerning the reasons for these changes in behaviour, see Bug#17619.

For SQL statements used to create, alter, and drop events, see 項19.2. 「Event Scheduler Syntax」.

MySQL 5.1.6 and later provides an EVENTS table in the INFORMATION_SCHEMA database. This table can be queried to obtian information about scheduled events which have been defined on the server. See 項19.3. 「Event Metadata」, and 項21.20. 「INFORMATION_SCHEMA EVENTS テーブル」, for more information.

For information regarding event scheduling and the MySQL privilege system, see 項19.5. 「The Event Scheduler and MySQL Privileges」.