Mysql事务隔离级别_MySQL-Mysql Tutorial-php.cn

Mysql事务隔离级别_MySQL

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Release： 2016-06-01 13:00:06

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scott@PROD>select * from dept1;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        50 OPERATIONS     BOSTON
        20 DBA            Bei Jing

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scott@PROD>update dept1 set deptno=21 where dname='DBA';

1 row updated.

scott@PROD>SELECT s.sid, s.serial#,
  2     CASE BITAND(t.flag, POWER(2, 28))
  3        WHEN 0 THEN 'READ COMMITTED'
  4        ELSE 'SERIALIZABLE'
  5     END AS isolation_level
  6  FROM v$transaction t 
  7  JOIN v$session s ON t.addr = s.taddr
  8  AND s.sid = sys_context('USERENV', 'SID');

       SID    SERIAL# ISOLATION_LEVE
---------- ---------- --------------
        41       5973 READ COMMITTED
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  Oracle数据库支持READ COMMITTED 和 SERIALIZABLE这两种事务隔离级别。 

  而Mysql支持READ UNCOMMITED,READ COMMITED,REPEATABLE READ,SERIALIZABLE四种事务隔离级别
  
  
    READ UNCOMMITTED（读取未提交内容） 

        在read uncommitted隔离级别，所有事物都可以”看到”未提交事物的执行结果。也就是脏读(读取未提交事务)
    
    READ COMMITED (读取提交内容) 

        ORACLE的默认隔离级别。一个事物开始时，只能”看见”已经提交事务所做的改变，一个事务从开始提交前，所做的任何数据改变都是不可见的，除非已经提交。这种隔离级别也就是不可重复读
    
    REPEATABLE READ (可重读) 

        repeatable read隔离级别解决了read uncommitted隔离级导致的问题。它确保同一事务的做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。简单来说，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薪行，当用户再读取该范围的数据行时，会发现有新的”还原”行。
    
    SERIALIZABLE (可串行化) 

        serializable是最高级别的隔离级，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。
  


Mysql默认的隔离级别是：

(mysql@localhost) [fandb]> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
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session A:
(mysql@localhost) [fandb]> begin;
Query OK, 0 rows affected (0.00 sec)

(mysql@localhost) [fandb]> update per1 set name='fan1' where id=1
    -> ;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

(mysql@localhost) [fandb]> select * from per1 limit 1;
+----+------+
| id | name |
+----+------+
|  1 | fan1 |
+----+------+
1 row in set (0.00 sec)
A会话更新一行
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session B:
(mysql@localhost) [fandb]> begin;
Query OK, 0 rows affected (0.00 sec)

(mysql@localhost) [fandb]> select * from per1 limit 1;
+----+------+
| id | name |
+----+------+
|  1 | fan  |
+----+------+
1 row in set (0.00 sec)
此时在B开始事务并查询，id=1的name列并没有变化
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session A:
(mysql@localhost) [fandb]> commit;
Query OK, 0 rows affected (0.00 sec)
接着A会话提交
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session B:
(mysql@localhost) [fandb]> select * from per1 limit 1;
+----+------+
| id | name |
+----+------+
|  1 | fan  |
+----+------+
1 row in set (0.00 sec)
在去B会话查询，还是没有变化
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(mysql@localhost) [fandb]> commit;
Query OK, 0 rows affected (0.00 sec)

(mysql@localhost) [fandb]> select * from per1 limit 1;
+----+------+
| id | name |
+----+------+
|  1 | fan1 |
+----+------+
1 row in set (0.00 sec)
只有当B会话事务结束，再次查询记录才会变化
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