mysqlInnoDB行锁的一点体会

mysql InnoDB行锁的一点体会

 

表结构如下 

Sql代码  

CREATE TABLE `20121015_t` (  

  `id` int(11) NOT NULL AUTO_INCREMENT,  

  `a` int(11) NOT NULL,  

  `b` int(11) NOT NULL,  

  PRIMARY KEY (`id`),  

  KEY `20121015_t_a` (`a`)  

) ENGINE=InnoDB  

  www.2cto.com  

现在只在列a上有索引 

在两个客户端分别执行 

Sql代码  

update 20121015_t set b=4 where a=1 and b=3;  

 

Sql代码  

update 20121015_t set b=2 where a=1 and b=1;  

  www.2cto.com  

第二个会等待超时 

[Err] 1205 - Lock wait timeout exceeded; try restarting transaction 

 

原来InnoDB不是只在最终要更新的行上加锁,而是在被扫描过的所有行上加锁.也就是说, 

如果执行update 20121015_t set b=2 where b=1;其实表里所有的行都被加锁了. 
 

他们官方文档说明如下 

A locking read, an UPDATE, or a DELETE generally set record locks on every index record that is scanned in the processing of the SQL statement. It does not matter whether there are WHERE conditions in the statement that would exclude the row. InnoDB does not remember the exact WHERE condition, but only knows which index ranges were scanned.