MySQL 两种锁操作

Posted 2022-01-04 ayann204

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！