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標(biāo)題索引
變量分類
變量設(shè)置
變量分類
變量主要分為全局變量和會(huì)話變量,全局變量是當(dāng)客戶端程序與服務(wù)器端建立會(huì)話后自動(dòng)集成部分全局變量的值,而會(huì)話變量是只對當(dāng)前變量有效,另外全局變量有些支持在線修改,有些則是只讀變量,只讀變量只允許在配置文件中加載值。
變量設(shè)置
1.全局變量設(shè)置
全局變量保存在數(shù)據(jù)庫information_schema的GLOBAL_VARIABLES表中,因此修改時(shí)可以通過配置文件加載時(shí)修改,部分變量也可通過在線修改,在線修改時(shí)通常有兩種命令方式,分別如下:
mysql>SET GLOBAL system_var_name=value;
mysql>SET @@global.system_var_name=value;
舉例說明如下:
MariaDB [(none)]> show global variables like 'transaction_prealloc_size'; +---------------------------+-------+ | Variable_name | Value | +---------------------------+-------+ | transaction_prealloc_size | 4096 | +---------------------------+-------+ 1 row in set (0.00 sec) MariaDB [(none)]> set global transaction_prealloc_size=8192; Query OK, 0 rows affected (0.00 sec) MariaDB [(none)]> show global variables like 'transaction_prealloc_size'; +---------------------------+-------+ | Variable_name | Value | +---------------------------+-------+ | transaction_prealloc_size | 8192 | +---------------------------+-------+ 1 row in set (0.00 sec) MariaDB [(none)]> set @@global.transaction_prealloc_size=4096; Query OK, 0 rows affected (0.00 sec) MariaDB [(none)]> show global variables like 'transaction_prealloc_size'; +---------------------------+-------+ | Variable_name | Value | +---------------------------+-------+ | transaction_prealloc_size | 4096 | +---------------------------+-------+ 1 row in set (0.00 sec)
2.會(huì)話變量設(shè)置
會(huì)話變量保存在數(shù)據(jù)庫information_schema的SESSION_VARIABLES表中,因此修改時(shí)可以通過配置文件加載時(shí)修改,部分變量也可通過在線修改,在線修改時(shí)通常有兩種命令方式,分別如下:
mysql>SET SESSION system_var_name=value;
mysql>SET @@session.system_var_name=value
MariaDB [(none)]> show session variables like 'wait_timeout'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | wait_timeout | 28800 | +---------------+-------+ 1 row in set (0.00 sec) MariaDB [(none)]> set session wait_timeout=30000; Query OK, 0 rows affected (0.00 sec) MariaDB [(none)]> show session variables like 'wait_timeout'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | wait_timeout | 30000 | +---------------+-------+ 1 row in set (0.00 sec) MariaDB [(none)]> set @@session.wait_timeout=28800; Query OK, 0 rows affected (0.00 sec) MariaDB [(none)]> show session variables like 'wait_timeout'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | wait_timeout | 28800 | +---------------+-------+ 1 row in set (0.00 sec)
3.會(huì)話變量轉(zhuǎn)全局變量
會(huì)話變量調(diào)整為全局變量時(shí),通常采用如下方法:
mysql> set @@session.wait_timeout=@@global.wait_timeout;
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