FLUSH TABLES WITH READ LOCK

会话1：

mysql> FLUSH TABLES WITH READ LOCK;
Query OK, 0 rows affected (0.18 sec)

mysql> show processlist;
+----+------+-----------------+------+---------+------+----------+------------------+
| Id | User | Host            | db   | Command | Time | State    | Info             |
+----+------+-----------------+------+---------+------+----------+------------------+
| 12 | root | localhost:48850 | NULL | Sleep   | 3603 |          | NULL             |
| 13 | root | localhost:48851 | test | Sleep   | 2665 |          | NULL             |
| 14 | root | localhost:49828 | NULL | Query   |    0 | starting | show processlist |
+----+------+-----------------+------+---------+------+----------+------------------+
3 rows in set (0.00 sec)

mysql> use test;
Database changed
mysql> create table tt( a int);
ERROR 1223 (HY000): Can‘t execute the query because you have a conflicting read lock

会话2：

mysql> use test;
Database changed
mysql> create table tt(a int);

会话1：

mysql> show processlist;
+----+------+-----------------+------+---------+------+------------------------------+------------------------+
| Id | User | Host            | db   | Command | Time | State                        | Info                   |
+----+------+-----------------+------+---------+------+------------------------------+------------------------+
| 12 | root | localhost:48850 | NULL | Sleep   | 3712 |                              | NULL                   |
| 13 | root | localhost:48851 | test | Sleep   | 2774 |                              | NULL                   |
| 14 | root | localhost:49828 | test | Query   |    0 | starting                     | show processlist       |
| 15 | root | localhost:49833 | test | Query   |    7 | Waiting for global read lock | create table tt(a int) |
+----+------+-----------------+------+---------+------+------------------------------+------------------------+
4 rows in set (0.00 sec)

1.当退去会话1后，就会自动释放锁,会话2,就可执行
2.unlock table