MySQL系统变量(systemvariables)_MySQL

1、什么是系统变量

对于有关涉及到size的设置值，可以使用后缀K、M或G分别表示千字节、兆字节或gigabytes，不区分大小写。

--当前的版本
mysql> show variables like &＃39;version%&＃39;;
+-------------------------+------------------------------+
| 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: &＃39;SET&＃39;
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;  --该命令会输出当前系统全部系统变量

--查看sort_buffer
mysql> show variables like &＃39;sort_buffer%&＃39;;
+------------------+---------+
| Variable_name    | Value   |
+------------------+---------+
| sort_buffer_size | 2097152 |
+------------------+---------+

--在省略global与session关键字的情形下为session级别
mysql> set sort_buffer_size=1024*1024*4;   --设置为4M

mysql> show variables like &＃39;sort_buffer%&＃39;;
+------------------+---------+
| Variable_name    | Value   |
+------------------+---------+
| sort_buffer_size | 4194304 |
+------------------+---------+

--恢复到缺省值
mysql> set sort_buffer_size=default;

mysql> show variables like &＃39;sort_buffer%&＃39;;
+------------------+---------+
| Variable_name    | Value   |
+------------------+---------+
| sort_buffer_size | 2097152 |
+------------------+---------+

3、全局与会话级别设置示例

--如何设置隔离级别
mysql> help isolation
Name: &＃39;ISOLATION&＃39;
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 &＃39;%isolation%&＃39;;
+---------------+-----------------+
| 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 &＃39;%isolation%&＃39;;
+---------------+----------------+
| 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 &＃39;%isolation%&＃39;;
+---------------+-----------------+
| 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 &＃39;%isolation%&＃39;;
+---------------+----------------+
| 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 &＃39;%isolation%&＃39;;
+---------------+--------------+
| Variable_name | Value        |
+---------------+--------------+
| tx_isolation  | SERIALIZABLE |
+---------------+--------------+

--在fred中全局的也变成了SERIALIZABLE
fred@localhost[(none)]> show global variables like &＃39;%isolation%&＃39;;
+---------------+--------------+
| 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 &＃39;%isolation%&＃39;;
+---------------+--------------+
| 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 &＃39;vari_name&＃39;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=&＃39;tx_isolation&＃39;;
+----------------+
| 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=&＃39;tx_isolation&＃39;;
+---------------+----------------+
| VARIABLE_NAME | VARIABLE_VALUE |
+---------------+----------------+
| TX_ISOLATION  | SERIALIZABLE   |
+---------------+----------------+

5、总结

b、检索设置

c、其他注意事项