[置顶] MySQL 表锁以及FLUSH TABLES操作
创建测试表t1, t2
use test;
CREATE TABLE `t1` ( `i` int(11) NOT NULL DEFAULT '0', PRIMARY KEY (`i`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `t2` ( `i` int(255) NOT NULL, PRIMARY KEY (`i`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;先来测试一下read锁
SESSION 1
mysql> lock table t1 read;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from t1 limit 1; +---+
| i | +---+
| 1 | +---+
1 row in set (0.02 sec)
mysql> insert into t1 values(104);
ERROR 1099 (HY000): Table 't1' was locked with a READ lock and can't be updated
mysql> update t1 set i=105 where i=104;
ERROR 1099 (HY000): Table 't1' was locked with a READ lock and can't be updated可见，对于加了读锁的表，在执行加读锁的session中可读取该表，但不能插入和更新，当然也不能进行删除。
mysql> select * from t2 limit 1;
ERROR 1100 (HY000): Table 't2' was not locked with LOCK TABLES
mysql> insert into t2 values(106);
ERROR 1100 (HY000): Table 't2' was not locked with LOCK TABLES
mysql> update t2 set i=105 where i=104;
ERROR 1100 (HY000): Table 't2' was not locked with LOCK TABLES对于没有加锁的表，不能在执行加锁的session中对表进行访问（包括增删改查）
SESSION 2
mysql> select * from t1 limit 1; +---+
| i | +---+
| 1 | +---+
1 row in set (0.04 sec)
mysql> select * from t2 limit 1; +---+
| i | +---+
| 1 | +---+
1 row in set (0.01 sec)
mysql> insert into t1 values(105);
ERROR 1099 (HY000): Table 't1' was locked with a READ lock and can't be updated可见，对于加了读锁的表，在不同的SESSION中不可以进行插入操作（更新和删除同理）。而且可以对任意的表进行读取。
再来测试一下write锁(记得先在SESSION 1解锁刚刚被加锁的表)
SESSION 1
mysql> lock table t1 write;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t1 limit 1; +---+
| i | +---+
| 1 | +---+
1 row in set (0.03 sec)
mysql> insert into t1 values(1000000);
Query OK, 1 row affected (0.03 sec)
mysql> update t1 set i=10000001 where i=1000000;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> delete from t1 where i=10000001;
Query OK, 1 row affected (0.03 sec)SESSION 2
mysql> select * from t1 limit 1;可见，若SESSION中对表加了写锁，则同一SESSION中对该表的增删改查均可进行，但其他SESSION中的读取和修改操作会被阻塞，直至表锁被释放。
接下来，来了解加锁前表上有尚未执行完成的query会怎样？
SESSION 1(记得先解锁刚刚被加锁的表)
mysql> select i,sleep(60) from t1 limit 1; +---+-----------+
| i | sleep(60) | +---+-----------+
| 1 | 0 | +---+-----------+
1 row in set (1 min 0.03 sec)SESSION 2
mysql> lock table t1 read;
Query OK, 0 rows affected (0.00 sec)
mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)
mysql> lock table t1 write ;
Query OK, 0 rows affected (9.15 sec)可见，表上有尚未完成的查询操作时可以加读锁，但写锁会被阻塞。
SESSION 1
mysql> begin;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into t1 values(1000000);
Query OK, 1 row affected (13.33 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)SESSION 2
mysql> lock table t1 read;
Query OK, 0 rows affected (0.00 sec)
mysql> unlock tables;
Query OK, 0 rows affected (0.00 sec)
mysql> lock table t1 write ;可见，表上有完成但尚未提交的插入操作时可以立刻获取读锁，但写锁会被阻塞。
接下来，来了解FLUSH TABLES时表上有尚未执行完成的查询会怎样？
SESSION 1
mysql> select i,sleep(60) from t1 limit 1;SESSION 2
mysql> flush tables t1;可见，由于将要被flush的表上有查询尚未完成，因此flush tables 操作被阻塞，直至所有表上的操作完成，flush tables操作才得以完成
SESSION 3
mysql> select * from t1 limit 1;由于flush tables t1被阻塞，导致后续其他session中的对于该表的查询被阻塞
SESSION 4
mysql> select * from processlist where db='test'; +------+------+-----------+------+---------+------+-------------------------+------------------------------------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO | +------+------+-----------+------+---------+------+-------------------------+------------------------------------+
| 8827 | root | localhost | test | Query | 31 | Waiting for table flush | select * from t1 limit 1 |
| 8825 | root | localhost | test | Query | 60 | User sleep | select i,sleep(60) from t1 limit 1 |
| 8826 | root | localhost | test | Query | 45 | Waiting for table flush | flush tables t1 | +------+------+-----------+------+---------+------+-------------------------+------------------------------------+另外的一个session中看到的线程状态。直至ID为8825的线程执行完了SQL后续的flush tables动作才得以执行，继而后续的select操作才顺利完成。
可见执行flush tables操作或者隐含包含flush tables的操作时要小心谨慎。
mysql> select * from processlist where db='test'; +------+------+-----------+------+---------+------+-------+------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO | +------+------+-----------+------+---------+------+-------+------+
| 8827 | root | localhost | test | Sleep | 245 | | NULL |
| 8825 | root | localhost | test | Sleep | 274 | | NULL |
| 8826 | root | localhost | test | Sleep | 259 | | NULL | +------+------+-----------+------+---------+------+-------+------+