标签：2-2
两种修改隔离级别的方法
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
方法1：
用户可以用SET TRANSACTION语句改变单个会话或者所有新进连接的隔离级别。它的语法如下：
SET [SESSION | GLOBAL] TRANSACTIONISOLATION LEVEL{READ UNCOMMITTED| READ COMMITTED | REPEATABLE READ| SERIALIZABLE}
注意：默认的行为（不带session和global）是为下一个（未开始）事务设置隔离级别。如果你使用GLOBAL关键字，语句在全局对从那点开始创建的所有新连接（除了不存在的连接）设置默认事务级别。你需要SUPER权限来做这个。使用SESSION关键字为将来在当前连接上执行的事务设置默认事务级别。任何客户端都能自由改变会话隔离级别（甚至在事务的中间），或者为下一个事务设置隔离级别。 你可以用下列语句查询全局和会话事务隔离级别：
SELECT @@global.tx_isolation;
SELECT @@session.tx_isolation;
SELECT @@tx_isolation;
网上有人使用set tx_isolation命令：
mysql> settx_isolation=‘read-committed‘;Query OK, 0 rows affected (0.00sec)+----------------+| @@tx_isolation |+----------------+| READ-COMMITTED|+----------------+1 row in set (0.00 sec)
mysql> select@@session.tx_isolation;+------------------------+| @@session.tx_isolation|+------------------------+|READ-COMMITTED |+------------------------+1 row in set (0.00 sec)事务隔离变了。网上还有人这样写set @@tx_isolation命令，但这个命令是有问题的。
mysql> set @@tx_isolation=‘read-committed‘;QueryOK, 0 rows affected (0.00 sec)
mysql> select@@session.tx_isolation;+------------------------+| @@session.tx_isolation|+------------------------+| REPEATABLE-READ |+------------------------+1 row in set (0.00 sec)
mysql> select @@tx_isolation;+-----------------+|@@tx_isolation |+-----------------+| REPEATABLE-READ |+-----------------+1row in set (0.00 sec)session事物的隔离级别并没有改变。
方法2：
[sql]viewplaincopy
1. mysql> select version();
2. +------------+
3. | version() |
4. +------------+
5. | 5.1.52-log |
6. +------------+
7. 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]viewplaincopy
1. mysql> select @@global.tx_isolation,@@tx_isolation;
2. +-----------------------+-----------------+
3. | @@global.tx_isolation | @@tx_isolation |
4. +-----------------------+-----------------+
5. | REPEATABLE-READ | REPEATABLE-READ |
6. +-----------------------+-----------------+
7. 1 row in set (0.00 sec)
更改会话级的隔离级别
[sql]viewplaincopy
1. Session 1:
2. mysql> set session tx_isolation=‘read-uncommitted‘;
3. Query OK, 0 rows affected (0.00 sec)
4. mysql> select @@global.tx_isolation,@@tx_isolation;
5. +-----------------------+------------------+
6. | @@global.tx_isolation | @@tx_isolation |
7. +-----------------------+------------------+
8. | REPEATABLE-READ | READ-UNCOMMITTED |
9. +-----------------------+------------------+
10. 1 row in set (0.00 sec)
11.
12.
13. Session 2:
14. mysql> select @@global.tx_isolation, @@tx_isolation;
15. +-----------------------+-----------------+
16. | @@global.tx_isolation | @@tx_isolation |
17. +-----------------------+-----------------+
18. | REPEATABLE-READ | REPEATABLE-READ |
19. +-----------------------+-----------------+
20. 1 row in set (0.00 sec)
更改系统级的隔离级别
[sql]viewplaincopy
1. Session 1:
2. mysql> set global tx_isolation=‘read-uncommitted‘;
3. Query OK, 0 rows affected (0.00 sec)
4. mysql> select @@global.tx_isolation,@@tx_isolation;
5. +-----------------------+------------------+
6. | @@global.tx_isolation | @@tx_isolation |
7. +-----------------------+------------------+
8. | READ-UNCOMMITTED | READ-UNCOMMITTED |
9. +-----------------------+------------------+
10. 1 row in set (0.00 sec)
11.
12. Session 2:
13. mysql> select @@global.tx_isolation, @@tx_isolation;
14. +-----------------------+-----------------+
15. | @@global.tx_isolation | @@tx_isolation |
16. +-----------------------+-----------------+
17. | READ-UNCOMMITTED | REPEATABLE-READ |
18. +-----------------------+-----------------+
19. 1 row in set (0.00 sec)
关闭SQL语句的自动提交
[sql]viewplaincopy
1. mysql> set autocommit=off;
2. Query OK, 0 rows affected (0.00 sec)
查看SQL语句自动提交是否关闭
[plain]viewplaincopy
1. mysql> show variables like ‘autocommit‘;
2. +---------------+-------+
3. | Variable_name | Value |
4. +---------------+-------+
5. | autocommit | OFF |
6. +---------------+-------+
7. 1 row in set (0.00 sec)
不重启修改MySQL的方法
在线修改mysql的系统变量：
mysql> show variables like ‘log_slave_updates‘; +-------------------+-------+|Variable_name |Value |+-------------------+-------+| log_slave_updates | ON |+-------------------+-------+1 row in set (0.00 sec)
mysql> set global log_slave_updates=0; ERROR1193 (HY000): Unknown system variable ‘log_slave_updates‘
mysql> system gdb -p $(pidof mysqld) -ex"set opt_log_slave_updates=0" -batchExcess command line argumentsignored. (9922 ...)19885: No such file or directory.(no debugging symbolsfound)Using host libthread_db library "/lib64/libthread_db.so.1".(nodebugging symbols found)(no debugging symbols found)(no debugging symbolsfound)(no debugging symbols found)[Thread debugging using libthread_dbenabled][New Thread 47873757603968 (LWP 12328)][New Thread 1166383424 (LWP12341)][New Thread 1166117184 (LWP 12339)][New Thread 1092294976 (LWP12338)][New Thread 1165850944 (LWP 12337)][New Thread 1155361088 (LWP12336)][New Thread 1144871232 (LWP 12335)][New Thread 1134381376 (LWP12333)][New Thread 1123891520 (LWP 12332)][New Thread 1113401664 (LWP 12331)][NewThread 1102911808 (LWP 12330)](no debugging symbols found)(no debugging symbolsfound)(no debugging symbols found)(no debugging symbols found)(no debuggingsymbols found)(no debugging symbols found)(no debugging symbols found)(nodebugging symbols found)0x00000039314cb332 in select () from /lib64/libc.so.6
mysql> show variables like ‘log_slave_updates‘; +-------------------+-------+|Variable_name |Value |+-------------------+-------+| log_slave_updates | OFF |+-------------------+-------+1row in set (0.00 sec)
MySQL系统参数设置
http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html
InonoDB参数设置
http://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html
演练参数
innodb_buffer_pool_size
标签：2-2
原文地址：http://yanfeilai528.blog.51cto.com/1079360/1955585
