MYSQL的伪行级锁_MySQL

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MYSQL的伪行级锁 之前一直以为mysql的innodb引擎所支持的行级锁和oracle，postgresql是一样的，是对数据行上加锁。但其实是不一样的，理解不一样，对mysql的锁机制就容易产生误解。innodb的行级锁实际上是基于索引项来锁定的。以下是验证测试过程 一.数据准备mysql> use test;Database changedmysql> show create table t_kenyon /G*************************** 1. row *************************** Table: t_kenyonCreate Table: CREATE TABLE `t_kenyon` ( `id` int(11) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec) mysql> set autocommit = 0;Query OK, 0 rows affected (0.00 sec) mysql> show variables like '%autocommit%';+---------------+-------+| Variable_name | Value |+---------------+-------+| autocommit | OFF |+---------------+-------+1 row in set (0.00 sec) mysql> select * from t_kenyon;+------+| id |+------+| 1 || 123 || 789 || 345 || 78 || 78 |+------+6 rows in set (0.00 sec)以上是测试表t_kenyon,设置提交方式为手动提交. 二.过程(开启两个session,分别设置autocommit=off) 1.session one updatemysql> update t_kenyon set id = 999 where id = 1;Query OK, 1 row affected (0.04 sec)Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_kenyon;+------+| id |+------+| 999 || 123 || 789 || 345 || 78 || 78 |+------+6 rows in set (0.00 sec)2.session two updatemysql> show variables like 'autocommit';+---------------+-------+| Variable_name | Value |+---------------+-------+| autocommit | OFF |+---------------+-------+1 row in set (0.00 sec) mysql> select * from t_kenyon;+------+| id |+------+| 1 || 123 || 789 || 345 || 78 || 78 |+------+6 rows in set (0.00 sec) mysql> update t_kenyon set id = 88888 where id = 345;第二个session更新的值是345，但是也一直被阻塞，直到session1被rollback或者commit,如果session1未做回滚或者提交，session2中的该阻塞在超出mysql的锁时间限制时自动回滚,该参数为innodb_lock_wait_timeout，默认值50秒 现象如下ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction加索引后的测试 3.session one update mysql> create index ind_kenyon on t_kenyon(id);Query OK, 0 rows affected (28.58 sec)Records: 0 Duplicates: 0 Warnings: 0 mysql> update t_kenyon set id = 999 where id = 1;Query OK, 1 row affected (0.03 sec)Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_kenyon;+------+| id |+------+| 78 || 78 || 123 || 345 || 789 || 999 |+------+6 rows in set (0.00 sec)4.session two update mysql> select * from t_kenyon;+------+| id |+------+| 1 || 78 || 78 || 123 || 345 || 789 |+------+6 rows in set (0.00 sec) mysql> update t_kenyon set id = 7777 where id = 345;Query OK, 1 row affected (0.03 sec)Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from t_kenyon;+------+| id |+------+| 1 || 78 || 78 || 123 || 789 || 7777 |+------+6 rows in set (0.00 sec)执行计划mysql> explain select * from t_kenyon where id = 345 /G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: t_kenyon type: refpossible_keys: ind_kenyon key: ind_kenyon key_len: 5 ref: const rows: 1 Extra: Using where; Using index1 row in set (0.00 sec)可以看到加了索引后，不同的数据更新并没有被阻塞，实现了真正意义上行锁 三.行级锁的扩展限制 参考：http:///database/201208/145888.html 作者 kenyon bitsCN.com