最近开发中遇到一些问题，这里记录下Mysql中的两种表级别的锁。
读锁：所有的会话只能进行SELECT语句查询
lock tables table_name READ;
写锁：只有当前会话能增删改查，其他会话无法任何操作
lock tables table_name WRITE
下面是个简单测试：
首先测试Write
下面先开启一个客户端进行如下操作：
mysql> show open tables from numtest; +----------+-------+--------+-------------+ | Database | Table | In_use | Name_locked | +----------+-------+--------+-------------+ | numtest | num | 1 | 0 | +----------+-------+--------+-------------+ 1 row in set (0.00 sec) mysql> lock tables num WRITE; Query OK, 0 rows affected (0.00 sec) mysql> show open tables from numtest; +----------+-------+--------+-------------+ | Database | Table | In_use | Name_locked | +----------+-------+--------+-------------+ | numtest | num | 1 | 0 | +----------+-------+--------+-------------+ 1 row in set (0.00 sec) mysql> select count(*) from num; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (0.00 sec)
然后开启客户端二操作：
mysql> select count(*) from numtest.num;
此时 客户端二会被lock
查看processlist得到验证：
mysql> show processlist; +--------+------+-----------+---------+---------+------+---------------------------------+----------------------------------+ | Id | User | Host | db | Command | Time | State | Info | +--------+------+-----------+---------+---------+------+---------------------------------+----------------------------------+ | 2 | root | localhost | numtest | Query | 0 | starting | show processlist | | 101003 | root | localhost | NULL | Query | 7 | Waiting for table metadata lock | select count(*) from numtest.num | +--------+------+-----------+---------+---------+------+---------------------------------+----------------------------------+ 2 rows in set (0.00 sec)
然后客户端A 释放锁：
mysql> unlock tables; Query OK, 0 rows affected (0.00 sec)
客户端二 执行OK（被锁了18s）。
mysql> select count(*) from numtest.num; +----------+ | count(*) | +----------+ | 1 | +----------+ 1 row in set (18.42 sec)
再测试Read Lock,同一个session操作。
mysql> lock tables num READ; Query OK, 0 rows affected (0.00 sec) mysql> mysql> mysql> insert into num values (1000,"2222"); ERROR 1099 (HY000): Table 'num' was locked with a READ lock and can't be updated mysql> show processlist; +--------+------+-----------+---------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +--------+------+-----------+---------+---------+------+----------+------------------+ | 2 | root | localhost | numtest | Query | 0 | starting | show processlist | | 101003 | root | localhost | NULL | Sleep | 2973 | | NULL | +--------+------+-----------+---------+---------+------+----------+------------------+ 2 rows in set (0.00 sec)
通过以上命令发现当前session也无法进行写操作
然后 释放锁，由client 2 进行写操作没问题
mysql> unlock tables; Query OK, 0 rows affected (0.00 sec) mysql> select count(*) from num; +----------+ | count(*) | +----------+ | 2 | +----------+ 1 row in set (0.00 sec) mysql> mysql> show processlist; +--------+------+-----------+---------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +--------+------+-----------+---------+---------+------+----------+------------------+ | 2 | root | localhost | numtest | Query | 0 | starting | show processlist | | 101003 | root | localhost | numtest | Sleep | 92 | | NULL | +--------+------+-----------+---------+---------+------+----------+------------------+
死锁的一般情况可以通过超时时间设定、或者kill掉被锁住的进程。
以上！