InnoDB的MVCC之(乐观锁),是通过在每行记录保存两个隐藏列来实现的。这两个列,一个是存创建时间,一个是删除时间,这里的时间指的是,系统版本号,并不是真正的时间值。
每开始一个新的事务,系统版本号都会自动递增。事务开始时刻的系统版本号会作为事务的版本号,用来和查询到的每行记录版本号比较。
下面看一个在REPEATABLE READ(可重复读)隔离级别下,MVCC的具体操作:
SELECT
InnoDB会根据以下两个条件检查每行记录:
1、InnoDB只查找版本小于或等于当前事务版本的数据行,这样可以确保事务读取的行,是在事务开始前就已经存在的,或者是事务自身插入或者修改过的数据。
2、行的删除版本要么未定义,要么大于当前事务的版本。这可以确保事务赢取到的行,在事务开始前未被删除。
只有符合上述两个条件的记录,能能返回做为查询结果。
INSERT
InnoDB为新插入的每一行保存当前系统版本号作为行版本号。
DELETE
InnoDB为删除的每一行保存当前系统版本号作为行删除标识。
UPDATE
InnoDB为插入一行新记录,保存当前系统版本号作为行版本号,同时保存当前系统版本号到原来的行作为删除标识。
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保存这两个额外的系统版本号,使大多数读操作都可以不用加锁。这样设计使得读数据操作很简单,性能很好。并且也能保证只会读取到符合标准的行。不足之处是每行记录都需要额外的存储空间,需要做更多的检查工作,以及一些额外的维护工作。
MVCC只在REPEATABLE READ(可重复读)和READ COMMITTED(提交读)两个隔离级别下工作。其他两个隔离级别都和MVCC不兼容。READ UNCOMMITTED(未提交读)总是读取最新的数据行,而SERIALIZBLE(可串行化)则会对所有读取的行加锁。

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