老男孩MySQL笔记 第二天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 Uncommitted2.Read Committed3.Repeatable Read4.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

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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.6mysql> 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