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)