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锁是计算机协调多个进程或线程并发访问某一资源的机制。在数据库中,除传统的
计算资源(如CPU、RAM、I/O等)的争用以外,数据也是一种供许多用户共享的资源。如何保证数据并发访问的一致性、有效性是所有数据库必须解决的一
个问题,锁冲突也是影响数据库并发访问性能的一个重要因素。从这个角度来说,锁对数据库而言显得尤其重要,也更加复杂。本章我们着重讨论MySQL锁机制
的特点,常见的锁问题,以及解决MySQL锁问题的一些方法或建议。 Mysql用到了很多这种锁机制,比如行锁,表锁等,读锁,写锁等,都是在做操作之前先上锁。这些锁统称为悲观锁(Pessimistic
Lock)。
MySQL锁概述
相对其他数据库而言,MySQL的锁机制比较简单,其最
显著的特点是不同的存储引擎支持不同的锁机制。比如,MyISAM和MEMORY存储引擎采用的是表级锁(table-level
locking);BDB存储引擎采用的是页面锁(page-level locking),但也支持表级锁;InnoDB存储引擎既支持行级锁(row-level
locking),也支持表级锁,但默认情况下是采用行级锁。 表级锁:开销小,加锁快;不会出现死锁;锁定粒度大,发生锁冲突的概率最高,并发度最低。 行级锁: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低,并发度也最高。 页面锁:开销和加锁时间界于表锁和行锁之间;会出现死锁;锁定粒度界于表锁和行锁之间,并发度一般 从上述特点可见,很难笼统地说哪种锁更好,只能就具体应用的特点来说哪种锁更合适!仅从锁的角度
来