MySQL中的事件调度器EVENT是怎样的

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MySQL中的事件调度器EVENT，可以在数据库里按照设定的时间周期触发某些操作，类似于定时任务机制。

MySQL中的事件调度器，EVENT，也叫定时任务，类似于Unix crontab或Windows任务调度程序。

EVENT由其名称和所在的schema唯一标识。

EVENT根据计划执行特定操作。操作由SQL语句组成，语句可以是BEGIN…END语句块。EVENT可以是一次性的，也可以是重复性的。一次性EVENT只执行一次，周期性EVENT以固定的间隔重复其操作，并且可以为周期性EVENT指定开始日期和时间、结束日期和时间。（默认情况下，定期EVENT在创建后立即开始，并无限期地继续，直到它被禁用或删除。）

EVENT由一个特殊的事件调度器线程执行，用SHOW PROCESSLIST可以查看。

root@database-one 13:44:  [gftest]> show variables like '%scheduler%';+-----------------+-------+| Variable_name   | Value |+-----------------+-------+| event_scheduler | OFF   |+-----------------+-------+1 row in set (0.01 sec)root@database-one 13:46:  [gftest]> show processlist;+--------+------+----------------------+-----------+---------+------+----------+------------------+| Id     | User | Host                 | db        | Command | Time | State    | Info             |+--------+------+----------------------+-----------+---------+------+----------+------------------+......+--------+------+----------------------+-----------+---------+------+----------+------------------+245 rows in set (0.00 sec)root@database-one 13:46:  [gftest]> set global event_scheduler=1;Query OK, 0 rows affected (0.00 sec)root@database-one 13:47:  [gftest]> show variables like '%scheduler%';+-----------------+-------+| Variable_name   | Value |+-----------------+-------+| event_scheduler | ON    |+-----------------+-------+1 row in set (0.01 sec)root@database-one 13:47:  [gftest]> show processlist;+--------+-----------------+----------------------+-----------+---------+------+------------------------+------------------+| Id     | User            | Host                 | db        | Command | Time | State                  | Info             |+--------+-----------------+----------------------+-----------+---------+------+------------------------+------------------+......| 121430 | event_scheduler | localhost            | NULL      | Daemon  |   33 | Waiting on empty queue | NULL             |......+--------+-----------------+----------------------+-----------+---------+------+------------------------+------------------+246 rows in set (0.01 sec)

可以看到，默认情况下，MySQL的EVENT没有打开，通过设置event_scheduler参数来打开或者关闭EVENT。打开后就会多一个event_scheduler，这个就是事件调度器线程。

除了打开和关闭，还可以禁用，要禁用EVENT，请使用以下两种方法之一：

• 启动MySQL时用命令行参数

--event-scheduler=DISABLED

• 在MySQL配置文件中配置参数

event_scheduler=DISABLED

MySQL 5.7中创建EVENT的完整语法如下：

CREATE    [DEFINER = user]    EVENT    [IF NOT EXISTS]    event_name    ON SCHEDULE schedule    [ON COMPLETION [NOT] PRESERVE]    [ENABLE | DISABLE | DISABLE ON SLAVE]    [COMMENT 'string']    DO event_body;schedule:    AT timestamp [+ INTERVAL interval] ...  | EVERY interval    [STARTS timestamp [+ INTERVAL interval] ...]    [ENDS timestamp [+ INTERVAL interval] ...]interval:    quantity {YEAR | QUARTER | MONTH | DAY | HOUR | MINUTE |              WEEK | SECOND | YEAR_MONTH | DAY_HOUR | DAY_MINUTE |              DAY_SECOND | HOUR_MINUTE | HOUR_SECOND | MINUTE_SECOND}

详细说明可以参考官网 https://dev.mysql.com/doc/refman/5.7/en/create-event.html

我们通过一个实例来验证下。
1）创建一张表。

root@database-one 13:47:  [gftest]> create table testevent(id int auto_increment primary key,create_time datetime);Query OK, 0 rows affected (0.01 sec)root@database-one 13:50:  [gftest]> select * from testevent;Empty set (0.00 sec)

2）创建一个EVENT，每3秒往表中插一条记录。

root@database-one 13:50:  [gftest]> create event insert_date_testevent on schedule every 3 second do     -> insert into testevent(create_time) values(now());Query OK, 0 rows affected (0.01 sec)root@database-one 13:53:  [gftest]> show events \G*************************** 1. row ***************************                  Db: gftest                Name: insert_date_testevent             Definer: root@%           Time zone: +08:00                Type: RECURRING          Execute at: NULL      Interval value: 3      Interval field: SECOND              Starts: 2020-03-26 13:53:10                Ends: NULL              Status: ENABLED          Originator: 1303306character_set_client: utf8collation_connection: utf8_general_ci  Database Collation: utf8_general_ci1 row in set (0.00 sec)

3）过一会，去表中查询数据。

root@database-one 13:53:  [gftest]> select * from testevent;+----+---------------------+| id | create_time         |+----+---------------------+|  1 | 2020-03-26 13:53:10 ||  2 | 2020-03-26 13:53:13 ||  3 | 2020-03-26 13:53:16 ||  4 | 2020-03-26 13:53:19 ||  5 | 2020-03-26 13:53:22 ||  6 | 2020-03-26 13:53:25 ||  7 | 2020-03-26 13:53:28 ||  8 | 2020-03-26 13:53:31 ||  9 | 2020-03-26 13:53:34 || 10 | 2020-03-26 13:53:37 || 11 | 2020-03-26 13:53:40 || 12 | 2020-03-26 13:53:43 || 13 | 2020-03-26 13:53:46 || 14 | 2020-03-26 13:53:49 || 15 | 2020-03-26 13:53:52 || 16 | 2020-03-26 13:53:55 |+----+---------------------+16 rows in set (0.00 sec)

从表里数据可以看到，创建的插数定时任务已经在正常运行了。

EVENT的详细信息除了用show event命令，还可以从mysql.event或information_schema.events中查询，也可以用show create event命令查看。

root@database-one 00:09:  [gftest]> select * from mysql.event \G*************************** 1. row ***************************                  db: gftest                name: insert_date_testevent                body: insert into testevent(create_time) values(now())             definer: root@%          execute_at: NULL      interval_value: 3      interval_field: SECOND             created: 2020-03-26 13:53:10            modified: 2020-03-26 13:53:10       last_executed: 2020-03-26 16:09:37              starts: 2020-03-26 05:53:10                ends: NULL              status: ENABLED       on_completion: DROP            sql_mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION             comment:           originator: 1303306           time_zone: +08:00character_set_client: utf8collation_connection: utf8_general_ci        db_collation: utf8_general_ci           body_utf8: insert into testevent(create_time) values(now())1 row in set (0.00 sec)root@database-one 00:09:  [gftest]> select * from information_schema.events \G*************************** 1. row ***************************       EVENT_CATALOG: def        EVENT_SCHEMA: gftest          EVENT_NAME: insert_date_testevent             DEFINER: root@%           TIME_ZONE: +08:00          EVENT_BODY: SQL    EVENT_DEFINITION: insert into testevent(create_time) values(now())          EVENT_TYPE: RECURRING          EXECUTE_AT: NULL      INTERVAL_VALUE: 3      INTERVAL_FIELD: SECOND            SQL_MODE: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION              STARTS: 2020-03-26 13:53:10                ENDS: NULL              STATUS: ENABLED       ON_COMPLETION: NOT PRESERVE             CREATED: 2020-03-26 13:53:10        LAST_ALTERED: 2020-03-26 13:53:10       LAST_EXECUTED: 2020-03-27 00:10:22       EVENT_COMMENT:           ORIGINATOR: 1303306CHARACTER_SET_CLIENT: utf8COLLATION_CONNECTION: utf8_general_ci  DATABASE_COLLATION: utf8_general_ci1 row in set (0.02 sec)root@database-one 00:10:  [gftest]> show create event insert_date_testevent \G*************************** 1. row ***************************               Event: insert_date_testevent            sql_mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION           time_zone: +08:00        Create Event: CREATE DEFINER=`root`@`%` EVENT `insert_date_testevent` ON SCHEDULE EVERY 3 SECOND STARTS '2020-03-26 13:53:10' ON COMPLETION NOT PRESERVE ENABLE DO insert into testevent(create_time) values(now())character_set_client: utf8collation_connection: utf8_general_ci  Database Collation: utf8_general_ci1 row in set (0.00 sec)

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