MySQL metadata锁实验
在对已经存在的表执行DDL语句（例如更改表结构、DROP表等操作)的时候，如果有与这个表相关联的事务没有提交，就会产生metadata锁等待。
这个时候，需要提交与这张表相关联的事务，才会将metadata锁释放。
创建一张表
mysql> CREATE TABLE `travelrecord` (
-> `id` bigint(20) NOT NULL,
-> `user_id` varchar(100) DEFAULT NULL,
-> `traveldate` date DEFAULT NULL,
-> `fee` decimal(10,0) DEFAULT NULL,
-> `days` int(11) DEFAULT NULL,
-> PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=gbk;
Query OK, 0 rows affected (0.09 sec)
会话一
开启一个事务，查询新创建的表
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from travelrecord;
Empty set (0.00 sec)
会话二
drop刚刚创建的表，会发生锁等待
mysql> drop table travelrecord;
会话一
查询会话
mysql> show processlist;
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
| 3 | root | 127.0.0.1:34875 | sale | Query | 0 | init | show processlist |
| 24 | root | 127.0.0.1:34876 | sale | Query | 1058 | Waiting for table metadata lock | drop table travelrecord |
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
2 rows in set (0.00 sec)
提交事务
mysql> commit;
Query OK, 0 rows affected (0.03 sec)
会话二
锁释放，DROP语句顺利执行
mysql> drop table travelrecord;
Query OK, 0 rows affected (58 min 40.02 sec)
数据库中没有了锁
mysql> show processlist;
+----+------+-----------------+------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+-------+------------------+
| 3 | root | 127.0.0.1:34875 | sale | Sleep | 76 | | NULL |
| 24 | root | 127.0.0.1:34876 | sale | Query | 0 | init | show processlist |
+----+------+-----------------+------+---------+------+-------+------------------+
2 rows in set (0.00 sec)
这个时候，需要提交与这张表相关联的事务，才会将metadata锁释放。
创建一张表
mysql> CREATE TABLE `travelrecord` (
-> `id` bigint(20) NOT NULL,
-> `user_id` varchar(100) DEFAULT NULL,
-> `traveldate` date DEFAULT NULL,
-> `fee` decimal(10,0) DEFAULT NULL,
-> `days` int(11) DEFAULT NULL,
-> PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=gbk;
Query OK, 0 rows affected (0.09 sec)
会话一
开启一个事务，查询新创建的表
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from travelrecord;
Empty set (0.00 sec)
drop刚刚创建的表，会发生锁等待
mysql> drop table travelrecord;
会话一
查询会话
mysql> show processlist;
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
| 3 | root | 127.0.0.1:34875 | sale | Query | 0 | init | show processlist |
| 24 | root | 127.0.0.1:34876 | sale | Query | 1058 | Waiting for table metadata lock | drop table travelrecord |
+----+------+-----------------+------+---------+------+---------------------------------+-------------------------+
2 rows in set (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.03 sec)
锁释放，DROP语句顺利执行
mysql> drop table travelrecord;
Query OK, 0 rows affected (58 min 40.02 sec)
数据库中没有了锁
mysql> show processlist;
+----+------+-----------------+------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+-------+------------------+
| 3 | root | 127.0.0.1:34875 | sale | Sleep | 76 | | NULL |
| 24 | root | 127.0.0.1:34876 | sale | Query | 0 | init | show processlist |
+----+------+-----------------+------+---------+------+-------+------------------+
2 rows in set (0.00 sec)
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/26506993/viewspace-2145655/ ，如需转载，请注明出处，否则将追究法律责任。
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