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

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)

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