如何確定數據庫讀取的哪一個，命令給出了答案

[root@hack home]# mysql --help |grep my.cnf
????????????????????? order of preference, my.cnf, $MYSQL_TCP_PORT,
/etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf ~/.my.cnf
[root@hack home]#

參數有的是為客戶端準備，有的是為服務器準備

[root@hack home]# cat /etc/my.cnf | grep -v "^#"

[client]

port=3306

[mysqld]
datadir=/opt/mysql/data

basedir = /usr/local/mysql
socket=/usr/local/mysql/mysql.sock
user=mysql
character_set_server=utf8
server_id = 12
join_buffer_size = 128M
sort_buffer_size = 2M
pid-file=/opt/mysql/data/mysqld.pid
[root@hack home]#

什么是參數？

這個就好比是鍵跟值得關系 如read_buffer_size=12M read_buffer_size就是鍵 12M就是指

分為兩類動態和靜態的（只讀），也可以分成是全局和局部的，動態的可以修改但是只會在mysql server的整個生命周期中起作用，下次重啟之后會重新根據參數配置文件加載，所以要想永久生效還是修改配置文件，局部只對當前session有效，全局的對所有重新上來的會話有效，動態修改參數使用set [global] variables=key。

查看參數

可以使用show variables like "variables_name" select @@session.variables_name select @@global.variables_name

如查看read_buffer_size

mysql> set read_buffer_size=524288;
Query OK, 0 rows affected (0.07 sec)

mysql> select @@session.read_buffer_size;
+----------------------------+
| @@session.read_buffer_size |
+----------------------------+
|???????????????????? 524288 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select @@global.read_buffer_size;
+---------------------------+
| @@global.read_buffer_size |
+---------------------------+
|??????????????????? 131072 |
+---------------------------+
1 row in set (0.00 sec)

mysql> show variables like "read_buffer_size%";
+------------------+--------+
| Variable_name??? | Value? |
+------------------+--------+
| read_buffer_size | 524288 |
+------------------+--------+
1 row in set (0.00 sec)

可以看到同樣設置了read_buffer_size但是session和global卻不一樣 因為我設置的時候沒有加global

如果更改靜態參數會有什么結果呢

mysql> set global datadir='/opt';
ERROR 1238 (HY000): Variable 'datadir' is a read only variable
mysql>