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Mysql 事务的隔离级别

2016-07-31 15:23 PHPer


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 Committed < Repeatable Read < Serializable。


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)




Thanks ~

 
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