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。
