MySQL系统变量(systemvariables)_MySQL

--当前的版本
mysql> show variables like 'version%';
+-------------------------+------------------------------+
| Variable_name | Value |
+-------------------------+------------------------------+
| version | 5.5.37 |
| version_comment | MySQL Community Server (GPL) |
| version_compile_machine | x86_64 |
| version_compile_os | Linux |
+-------------------------+------------------------------+

--获取有关set的帮助
mysql> help set
Name: 'SET'
Description:
Syntax:
SET variable_assignment [, variable_assignment] ...

variable_assignment:
 user_var_name = expr
 | [GLOBAL | SESSION] system_var_name = expr
 | [@@global. | @@session. | @@]system_var_name = expr

--查看全部系统变量
root@localhost[tempdb]> show variables; --该命令会

--如何设置隔离级别
mysql> help isolation
Name: 'ISOLATION'
Description:
Syntax:
SET [GLOBAL | SESSION] TRANSACTION ISOLATION LEVEL
 {
 REPEATABLE READ
 | READ COMMITTED
 | READ UNCOMMITTED
 | SERIALIZABLE
 }
 
--下面我们通过演示隔离级别来设置全局与session级别变量
--查看当前session级别的隔离方式
root@localhost[(none)]> show variables like '%isolation%';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+

--修改当前session级别的隔离方式为READ-COMMITTED
root@localhost[(none)]> set session transaction isolation level read committed;

root@localhost[(none)]> show variables like '%isolation%';
+---------------+----------------+
| Variable_name | Value |
+---------------+----------------+
| tx_isolation | READ-COMMITTED |
+---------------+----------------+

--另外的一个session ， 登录用户为fred
--当前sessioin级别继承全局隔离级别为REPEATABLE-READ
fred@localhost[(none)]> show variables like '%isolation%';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+

--在root会话中设置全局隔离级别为serializable
root@localhost[(none)]> set global transaction isolation level serializable;

--注意，在root会话中 session级别还是为READ-COMMITTED
root@localhost[(none)]> show variables like '%isolation%';
+---------------+----------------+
| Variable_name | Value |
+---------------+----------------+
| tx_isolation | READ-COMMITTED |
+---------------+----------------+

--在root会话中我可以看到全局的值已经变为SERIALIZABLE
root@localhost[(none)]> show global variables like '%isolation%';
+---------------+--------------+
| Variable_name | Value |
+---------------+--------------+
| tx_isolation | SERIALIZABLE |
+---------------+--------------+

--在fred中全局的也变成了SERIALIZABLE
fred@localhost[(none)]> show global variables like '%isolation%';
+---------------+--------------+
| Variable_name | Value |
+---------------+--------------+
| tx_isolation | SERIALIZABLE |
+---------------+--------------+

--从上面的演示来说，无论global级别如何设置，不会影响到当前session级别的设置

--下面我们使用一个新用户登录来看看全局设置是否影响新会话
robin@SZDB:~> mysql -urobin

--如下查询新会话的隔离级别等于全局的隔离级别
robin@localhost[(none)]> show variables like '%isolation%';
+---------------+--------------+
| Variable_name | Value |
+---------------+--------------+
| tx_isolation | SERIALIZABLE |
+---------------+--------------+

4、如何获取变量值

除了通过前面演示的使用show global|session variables like 'vari_name'方式之外，我们可以通过查询
information_schema数据中特定的表来获得这些变量的值。
通过查询数据information_schema的表global_variables

root@localhost[information_schema]> select variable_value from global_variables where 
 -> variable_name='tx_isolation';
+----------------+
| variable_value |
+----------------+
| SERIALIZABLE |
+----------------+

--Author: Leshami
--Blog : http://blog.csdn.net/leshami

root@localhost[information_schema]> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| SERIALIZABLE |
+-----------------------+

root@localhost[information_schema]> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| READ-COMMITTED |
+------------------------+

--下面查询session_variables结果与查询global_variables获得的值相同，究其原因还在进一步研究中
root@localhost[information_schema]> select * from session_variables where variable_name='tx_isolation';
+---------------+----------------+
| VARIABLE_NAME | VARIABLE_VALUE |
+---------------+----------------+
| TX_ISOLATION | SERIALIZABLE |
+---------------+----------------+