ホームページ >データベース >mysql チュートリアル >Mysql事务隔离级别_MySQL
<code class=" hljs brainfuck">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
<code class=" hljs vbnet">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
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:
<code class=" hljs asciidoc">(mysql@localhost) [fandb]> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+
<code class=" hljs asciidoc">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
<code class=" hljs asciidoc">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
<code class=" hljs css">session A: (mysql@localhost) [fandb]> commit; Query OK, 0 rows affected (0.00 sec) 接着A会话提交
<code class=" hljs asciidoc">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
<code class=" hljs asciidoc">(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