MySQL中的行级锁定示例详解

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前言

锁是在执行多线程时用于强行限定资源访问的同步机制，数据库锁根据锁的粒度可分为行级锁，表级锁和页级锁

行级锁

行级锁是mysql中粒度最细的一种锁机制，表示只对当前所操作的行进行加锁，行级锁发生冲突的概率很低，其粒度最小，但是加锁的代价最大。行级锁分为共享锁和排他锁。

特点：

开销大，加锁慢，会出现死锁；锁定粒度最小，发生锁冲突的概率最大，并发性也高；

实现原理：

InnoDB行锁是通过给索引项加锁来实现的，这一点mysql和oracle不同，后者是通过在数据库中对相应的数据行加锁来实现的，InnoDB这种行级锁决定，只有通过索引条件来检索数据，才能使用行级锁，否则，直接使用表级锁。特别注意:使用行级锁一定要使用索引

举个栗子：

创建表结构

CREATE TABLE `developerinfo` (
 `userID` bigint(20) NOT NULL,
 `name` varchar(255) DEFAULT NULL,
 `passWord` varchar(255) DEFAULT NULL,
 PRIMARY KEY (`userID`),
 KEY `PASSWORD_INDEX` (`passWord`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

插入数据

INSERT INTO `developerinfo` VALUES ('1', 'liujie', '123456');
INSERT INTO `developerinfo` VALUES ('2', 'yitong', '123');
INSERT INTO `developerinfo` VALUES ('3', 'tong', '123456');

（1）通过主键索引来查询数据库使用行锁

打开三个命令行窗口进行测试

命令行窗口1	命令行窗口2	命令行窗口3

mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set 
|mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;
等待
|mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '3' for update;
+--------+------+----------+
| userID | name | passWord |
+--------+------+----------+
| 3 | tong | 123456 |
+--------+------+----------+
1 row in set
|mysql> commit;
Query OK, 0 rows affected mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set

（2）查询非索引的字段来查询数据库使用行锁

打开两个命令行窗口进行测试

命令行窗口1	命令行窗口2

|mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where name = 'liujie' for update;
+--------+--------+----------+
userID name passWord
+--------+--------+----------+
1 liujie 123456
+--------+--------+----------+
1 row in set |mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where name = 'tong' for update;
等待|
mysql> commit;
Query OK, 0 rows affected mysql> select * from developerinfo where name = 'liujie' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set

##### （3）查询非唯一索引字段来查询数据库使用行锁锁住多行

mysql的行锁是针对索引假的锁，不是针对记录，所以可能会出现锁住不同记录的场景

打开三个命令行窗口进行测试

命令行窗口1 命令行窗口2 命令行窗口3

mysql> set autocommit=0;
Query OK, 0 rows affected
mysql> select * from developerinfo where password = '123456
' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
| 3 | tong | 123456 |
+--------+--------+----------+
2 rows in set mysql> set autocommit =0 ;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '1' for update;

等待

mysql> set autocommit = 0;
Query OK, 0 rows affected
mysql> select * from developerinfo where userid = '2
' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 2 | yitong | 123 |
+--------+--------+----------+
1 row in set
commit; mysql> select * from developerinfo where userid = '1' for update;
+--------+--------+----------+
| userID | name | passWord |
+--------+--------+----------+
| 1 | liujie | 123456 |
+--------+--------+----------+
1 row in set