MySQL隔离级别

http://blog.csdn.net/taylor_tao/article/details/7063639

事务具有ACID四种特性。

但是Isolation并发可能引起如下问题：

1.脏读

允许读取到未提交的脏数据。

2.不可重复读

如果你在时间点T1读取了一些记录，在T2时再想重新读取一次同样的这些记录时，这些记录可能已经被改变、或者消失不见。

3.幻读

解决了不重复读，保证了同一个事务里，查询的结果都是事务开始时的状态（一致性）。但是，如果另一个事务同时提交了新数据，本事务再更新时，就会“惊奇的”发现了这些新数据，貌似之前读到的数据是“鬼影”一样的幻觉。

由ANSI/ISO定义的SQL-92标准定义的四种隔离级别

1.Read Uncommitted

2.Read Committed

3.Repeatable Read

4.Serializable

隔离解别	脏读	不可重复读	幻读
Read Uncommitted	Y	Y	Y
Read Committed	N	Y	Y
Repeatable(default)	N	N	Y
Serializable	N	N	N

下面用Mysql数据库做一些小实验

Mysql 版本号

[sql] view plain copy

mysql> select version();

+------------+

| version() |

+------------+

| 5.1.52-log |

+------------+

1 row in set (0.00 sec)

查看InnoDB存储引擎 系统级的隔离级别 和 会话级的隔离级别

[sql] view plain copy

mysql> select @@global.tx_isolation,@@tx_isolation;

+-----------------------+-----------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+-----------------+

| REPEATABLE-READ | REPEATABLE-READ |

+-----------------------+-----------------+

1 row in set (0.00 sec)

更改会话级的隔离级别

[sql] view plain copy

Session 1:

mysql> set session tx_isolation='read-uncommitted';

Query OK, 0 rows affected (0.00 sec)

mysql> select @@global.tx_isolation,@@tx_isolation;

+-----------------------+------------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+------------------+

| REPEATABLE-READ | READ-UNCOMMITTED |

+-----------------------+------------------+

1 row in set (0.00 sec)

Session 2:

mysql> select @@global.tx_isolation, @@tx_isolation;

+-----------------------+-----------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+-----------------+

| REPEATABLE-READ | REPEATABLE-READ |

+-----------------------+-----------------+

1 row in set (0.00 sec)

更改系统级的隔离级别

[sql] view plain copy

Session 1:

mysql> set global tx_isolation='read-uncommitted';

Query OK, 0 rows affected (0.00 sec)

mysql> select @@global.tx_isolation,@@tx_isolation;

+-----------------------+------------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+------------------+

| READ-UNCOMMITTED | READ-UNCOMMITTED |

+-----------------------+------------------+

1 row in set (0.00 sec)

Session 2:

mysql> select @@global.tx_isolation, @@tx_isolation;

+-----------------------+-----------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+-----------------+

| READ-UNCOMMITTED | REPEATABLE-READ |

+-----------------------+-----------------+

1 row in set (0.00 sec)

关闭SQL语句的自动提交

[sql] view plain copy

mysql> set autocommit=off;

Query OK, 0 rows affected (0.00 sec)

查看SQL语句自动提交是否关闭

[plain] view plain copy

mysql> show variables like 'autocommit';

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| autocommit | OFF |

+---------------+-------+

1 row in set (0.00 sec)

建立实验表

[sql] view plain copy

mysql> create table tao (col1 tinyint unsigned, col2 varchar(20), primary key(col1));

Query OK, 0 rows affected (0.08 sec)

mysql> show create table tao \G;

*************************** 1. row ***************************

Table: tao

Create Table: CREATE TABLE `tao` (

`col1` tinyint(3) unsigned NOT NULL DEFAULT '0',

`col2` varchar(20) DEFAULT NULL,

PRIMARY KEY (`col1`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

演示脏读



更改隔离级别为Read Committed后，不存在脏读的问题。

[sql] view plain copy

mysql> set global tx_isolation='read-committed';

Query OK, 0 rows affected (0.00 sec)

mysql> set session tx_isolation='read-committed';

Query OK, 0 rows affected (0.00 sec)

演示不可重复读



更改隔离级别为Repeatable Read后，不存在不可重复读的问题。

[sql] view plain copy

mysql> set global tx_isolation='repeatable-read';

Query OK, 0 rows affected (0.00 sec)

mysql> set session tx_isolation='repeatable-read';

Query OK, 0 rows affected (0.00 sec)

幻读



更改隔离级别为完全串行化 Serializable 后，不存在幻读的问题。

[sql] view plain copy

mysql> set global tx_isolation='serializable';

Query OK, 0 rows affected (0.00 sec)

mysql> set session tx_isolation='serializable';

Query OK, 0 rows affected (0.00 sec)

在这种情况下，只允许一个事务在执行，其它事务必须等待这个事务执行完后才能执行。没有并发，只是单纯的串行。