mysql 的事件之 Waiting for table metadata lock

os:debian 8.2
mysql:5.5.49

nodeb 为 nodea 的一个slave，在nodea上删除一个无效的索引，发现nodeb上出现长时间的 Waiting for table metadata lock。
分析的过程如下：

环境

# lsb_release -a No LSB modules are available. Distributor ID: Debian Description: Debian GNU/Linux 8.2 (jessie) Release: 8.2 Codename: jessie # mysql

root@localhost> select version(); +---------------------+
| version() | +---------------------+
| 5.5.49-0+deb8u1-log | +---------------------+
1 row in set (0.00 sec)

nodea 操作

root@localhost> drop index idx_abc_x1 on abc;

nodeb 操作

root@localhost> show processlist; +---------+-------------+-------------------+--------+---------+----------+----------------------------------+------------------------------------------------------------------------------------------------------+
| Id | User | Host | db | Command | Time | State | Info | +---------+-------------+-------------------+--------+---------+----------+----------------------------------+------------------------------------------------------------------------------------------------------+
|      48 | system user |                   | NULL   | Connect | 10446048 | Waiting for master to send event | NULL                                                                                                 |
| 49 | system user | | EMdata | Connect | 1543 | Waiting for table metadata lock | DROP INDEX IDX_ABC_X1 on ABC ~~~
~~~ ~~~
| 2427727 | emsel       | 10.10.2.51:59835  | EMdata | Query   |      134 | Waiting for table metadata lock  | select * from abc

查询锁信息

root@localhost> SELECT * FROM information_schema.innodb_lock_waits;
root@localhost> SELECT * FROM information_schema.innodb_locks;
root@localhost> SELECT * FROM information_schema.innodb_trx;

因为slave都是select操作，和开发人员沟通后可以使用 kill trx_mysql_thread_id 杀掉进程。

所以尽量避免在高并发的时候做DDL操作。

lock_wait_timeout

还有一个参数 lock_wait_timeout 用来控制metadata lock的超时（单位为秒），允许的值范围为1到31536000（1年）。 默认值为31536000。

root@localhost> show global variables like 'lock_wait_timeout'; +-------------------+----------+
| Variable_name | Value | +-------------------+----------+
| lock_wait_timeout | 31536000 | +-------------------+----------+
1 row in set (0.00 sec)

参考：
https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_lock_wait_timeout