Mysql 从库上出现Waiting for table metadata lock 分析

背景：

业务反馈在主库上做了一个DDL操作，执行时间很快，但是从机上却一直报主从延迟告警。

分析：

1. 在主机上查看当前的活动线程，发现从机回放进程处于 Waiting for table metadata lock中，但是并没有发现任何其它可疑进程。

--------+-------------+-----------------+--------+---------+---------+----------------------------------+-----------------------------------+| Id     | User        | Host            | db     | Command | Time    | State                            | Info                              |+--------+-------------+-----------------+--------+---------+---------+----------------------------------+-----------------------------------+|      6 | system user |                 | NULL   | Connect | 3649022 | Waiting for master to send event | NULL                              ||      7 | system user |                 | testtt | Connect |       9 | Waiting for table metadata lock  | alter table v_t add column aa int || 181873 | vector      | 127.0.0.1:41634 | testtt | Query   |       0 | starting                         | show processlist                  |+--------+-------------+-----------------+--------+---------+---------+----------------------------------+-----------------------------------+

2. 查看系统的未提交事务，发现了一个业务进程开启了事务但是一直未提交。

+-----------------+--------+------+---------------------+---------------------+------------------------------------------------------------------------------------------------------+| host            | user   | time | trx_mysql_thread_id | trx_started         | left(p.info,100)                                                                                     |+-----------------+--------+------+---------------------+---------------------+------------------------------------------------------------------------------------------------------+| 127.0.0.1:41634 | vector |    0 |              181873 | 2020-04-23 20:41:29 | select p.host,p.user,p.time,t.trx_mysql_thread_id,t.trx_started,left(p.info,100) from information_sc |+-----------------+--------+------+---------------------+---------------------+------------------------------------------------------------------------------------------------------+

3. 手动KILL掉这个线程后，业务恢复正常

总结： 从机上也是可以通过设置set autocommit=0等方式来显示开启事务，从而阻塞从机回放线程的。