带你了解MySQL中的事件调度器EVENT

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: 1303306
character_set_client: utf8
collation_connection: utf8_general_ci
Database Collation: utf8_general_ci
1 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: +08:00
character_set_client: utf8
collation_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: 1303306
CHARACTER_SET_CLIENT: utf8
COLLATION_CONNECTION: utf8_general_ci
DATABASE_COLLATION: utf8_general_ci
1 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: utf8
collation_connection: utf8_general_ci
Database Collation: utf8_general_ci
1 row in set (0.00 sec)

来源：https://cloud.tencent.com/developer/article/1608131