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数据库事务的隔离级别

Jun 07, 2016 pm 03:58 PM
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数据库软件需要解决的主要问题为数据一致性和访问并发性之间的矛盾,在理论上人们总结出保证数据一致性需要解决的几种不确定性情况,并以此为基础提出了数据库事务的几种隔离级别。 首先,来看一下,解决数据一致性的过程中,方法会发生的几种不确定情况。

数据库软件需要解决的主要问题为“数据一致性”和“访问并发性”之间的矛盾,在理论上人们总结出保证数据一致性需要解决的几种不确定性情况,并以此为基础提出了数据库事务的几种隔离级别。

首先,来看一下,解决数据一致性的过程中,方法会发生的几种不确定情况。

脏读:

脏读又称无效数据的读出,是指在数据库访问的过程中,事务T1对某一值进行了修改,在没有提交之前,事务T2可以读取到事务T1的修改,由于T1事务还没有提交,因此T2读到的T1事务数据时无效的,即该数据为脏数据。

不可重复读:

在一个事务内,多次读同一个数据。在这个事务还没有结束时,另一个事务也访问该同一数据。那么,在第一个事务的两次读数据之间。由于第二个事务的修改,那么第一个事务读到的数据可能不一样,这样就发生了在一个事务内两次读到的数据是不一样的,因此称为不可重复读,即原始读取不可重复

通过将事务的隔离级别设置为repeatable read可以防止不可重复读,在oracle中,我们可以通过select for update达到同样的效果。

幻读:

情况1:

第一个事务对一个表中的数据进行了修改,比如这种修改涉及到表中的“全部数据行”。同时,第二个事务也修改这个表中的数据,这种修改是向表中插入“一行新数据”。那么,以后就会发生操作第一个事务的用户发现表中还有没有修改的数据行,就好象发生了幻觉一样.

情况2:

事务T1查询表t1(5条)和t2(10条)中的记录总数之和(应为15),在事务T1查询完表t1之后并在查询t2 之前,事务T2向t1和t2中分别插入两条记录,并提交,随后事务T1查询t2中的记录总数,并得出结果t1=5 t2=12,总记录数17,实际的记录总数应为15或者19,不存在17,就像出现了幻觉

解决幻读需要将事务的隔离级别设置为serialization,即串行化

为了避免出现上面的几种情况,在标准SQL规范中,定义了4个事务隔离级别。

Read Uncommited

为了避免允许脏读取,但不允许更新丢失。如果一个事务已经开始写数据,则另外一个事务则不允许同时进行写操作,但允许其他事务读此行数据

Read Commited

允许不可重复读取,但不允许脏读取。

repeatable read

禁止不可重复读取和脏读取,但是有时可能出现幻影数据

Serializable

提供严格的事务隔离。它要求事务序列化执行,事务只能一个接着一个地执行,但不能并发执行。

√: 可能出现 ×: 不会出现

  脏读 不可重复读 幻读
Read uncommitted
Read committed ×
Repeatable read × ×
Serializable × × ×
在oracle数据库中,存在READ COMMITED(默认),Serializable,readonly 三种隔离级别。不同的数据库软件对隔离级别的实现方式是不同的,实现的程度也是不同的,需要具体情况具体分析。
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