Mysql 事务的四种隔离级别-php手册-PHP中文网

开发工作中我们会使用到事务，那你们知道事务又分哪几种吗？
MYSQL标准定义了4类隔离级别，用来限定事务内外的哪些改变是可见的，哪些是不可见的。
低的隔离级一般支持更高的并发处理，并拥有更低的系统开销。
隔离级别由低到高：Read Uncommitted Read Uncommitted（读取未提交内容）

在该隔离级别，所有事务都可以看到其他未提交(commit)事务的执行结果。
本隔离级别很少用于实际应用，因为它的性能也不比其他级别好多少。
读取未提交的数据，也被称之为脏读（Dirty Read）。[窗口A]: mysql> set GLOBAL tx_isolation='READ-UNCOMMITTED'; Query OK, 0 rows affected (0.00 sec) mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) mysql> use test; Database changed mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from user; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | +----+------+ 2 rows in set (0.00 sec) [窗口B]: mysql> select @@tx_isolation; +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into test.user values (3, 'c'); Query OK, 1 row affected (0.00 sec) mysql> select * from user; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | | 3 | c | +----+------+ 3 rows in set (0.00 sec) //目前为止，窗口B并未commit; [窗口A]: mysql> select * from user ; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | | 3 | c | +----+------+ 3 rows in set (0.00 sec)Read Committed（读取提交内容）

这是大多数数据库系统的默认隔离级别（但不是MySQL默认的）。
它满足了隔离的简单定义：一个事务只能看见已经提交事务所做的改变。
这种隔离级别也支持所谓的不可重复读（NonrepeatableRead），因为同一事务的其他实例在该实例处理其间可能会有新的commit，所以同一 select 可能返回不同结果。[窗口A]： mysql> SET GLOBAL tx_isolation='READ-COMMITTED'; Query OK, 0 rows affected (0.00 sec) mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | +----+------+ 2 rows in set (0.00 sec) [窗口B]: mysql> SELECT @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | +----+------+ 2 rows in set (0.00 sec) mysql> delete from test.user where id=1; Query OK, 1 row affected (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | +----+------+ 1 row in set (0.00 sec) [窗口A]: mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 1 | a | | 2 | b | +----+------+ 2 rows in set (0.00 sec) [窗口B]: mysql> commit; Query OK, 0 rows affected (0.02 sec) [窗口A]: mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | +----+------+ 1 row in set (0.00 sec)Repeatable Read（可重读）

这是MySQL的默认事务隔离级别，它确保同一事务的多个实例在并发读取数据时，会看到同样的数据行。
不过理论上，这会导致另一个棘手的问题：幻读（Phantom Read）。
简单的说，幻读指当用户读取某一范围的数据行时，另一个事务又在该范围内插入了新行，当用户再读取该范围的数据行时，会发现有新的“幻影” 行。
InnoDB和Falcon存储引擎通过多版本并发控制（MVCC，Multiversion Concurrency Control）机制解决了该问题。[窗口A]: mysql> SET GLOBAL tx_isolation='REPEATABLE-READ'; Query OK, 0 rows affected (0.00 sec) mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) [窗口B]: mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) mysql> insert into test.user values (4, 'd'); Query OK, 1 row affected (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | | 4 | d | +----+------+ 2 rows in set (0.00 sec) [窗口A]: mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | +----+------+ 1 rows in set (0.00 sec) mysql> commit; Query OK, 0 rows affected (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | | 4 | d | +----+------+ 2 rows in set (0.00 sec)Serializable（序列化执行）

这是最高的隔离级别，它通过强制事务排序，使之不可能相互冲突，从而解决幻读问题。
简言之，它是在每个读的数据行上加上共享锁。在这个级别，可能导致大量的超时现象和锁竞争。 [窗口A]: mysql> SET GLOBAL tx_isolation='SERIALIZABLE'; Query OK, 0 rows affected (0.00 sec) mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | SERIALIZABLE | +----------------+ 1 row in set (0.00 sec) mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | | 4 | d | +----+------+ 2 rows in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into test.user values (5, 'e'); Query OK, 1 row affected (0.00 sec) [窗口B]: mysql> quit; Bye [root@vagrant-centos65 ~]# mysql -uroot -pxxxx(重新登录) mysql> SELECT @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | SERIALIZABLE | +----------------+ 1 row in set (0.00 sec) mysql> select * from test.user; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction [窗口A]: mysql> commit; Query OK, 0 rows affected (0.01 sec) [窗口B]: mysql> mysql> select * from test.user; +----+------+ | id | name | +----+------+ | 2 | b | | 4 | d | | 5 | e | +----+------+ 3 rows in set (0.00 sec)Thank ~

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Mysql 事务的四种隔离级别