mysql 的max_connections和max_user_connections 的区别

----查看max_user_connections 默认值

  MySQL> show variables like 'max_user_connections';
+----------------------+-------+
 | Variable_name        | Value |
 +----------------------+-------+
 | max_user_connections | 0     |
 +----------------------+-------+

1 row in set (0.00 sec)

---设置 max_user_connections
mysql> set  @@global.max_user_connections=1;

Query OK, 0 rows affected (0.03 sec)
mysql>  select @@max_user_connections;
 +------------------------+
 | @@max_user_connections |
 +------------------------+
 |                      1 |
 +------------------------+
 1 row in set (0.00 sec)
上面参数设置完后窗口，要重新登陆一下

root@dg ~]# mysql -uroot -pmysql
然后再开一个窗口登陆就会报如下错误：
root@dg ~]# mysql -uroot -pmysql
 Warning: Using a password on the command line interface can be insecure.

ERROR 1203 (42000): User root already has more than 'max_user_connections' active connections



另外在登陆一个用户u2，正常登陆，但是在登陆一个u2用户，就会报错：
root@dg mysql]# mysql -uu2 -pu2
 Warning: Using a password on the command line interface can be insecure.
 Welcome to the MySQL monitor.  Commands end with ; or g.
 Your MySQL connection id is 147
 Server version: 5.6.27-enterprise-commercial-advanced MySQL Enterprise Server - Advanced Edition (Commercial)
 Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
 Oracle is a registered trademark of Oracle Corporation and/or its
 affiliates. Other names may be trademarks of their respective
 owners.
 Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.


max_user_connections 针对用户设计的




下面我们来看看max_connections 的作用
mysql> select @@max_connections;
 +-------------------+
 | @@max_connections |
 +-------------------+
 |               151 |
 +-------------------+
 1 row in set (0.00 sec)
 mysql>  set  @@global.max_connections=2;
 Query OK, 0 rows affected (0.00 sec)
 mysql> select @@max_connections;
 +-------------------+
 | @@max_connections |
 +-------------------+
 |                 2 |
 +-------------------+
 1 row in set (0.00 sec)
上面参数设置完后窗口，要重新登陆一下
[root@dg mysql]# mysql -uroot -pmysql
在开一个窗口session2正常登陆：
[root@dg mysql]# mysql -uroot -pmysql
再开第三个窗口，session3 登陆的时候报错
[root@dg ~]# mysql -uroot -pmysql
 Warning: Using a password on the command line interface can be insecure.
 ERROR 1040 (HY000): Too many connections
 [root@dg ~]# mysql -uu2 -pu2
 Warning: Using a password on the command line interface can be insecure.
 ERROR 1040 (HY000): Too many connections


实验完毕后,改回原来的参数
mysql>  set  @@global.max_user_connections=0;
 Query OK, 0 rows affected (0.00 sec)

 mysql>  select @@max_user_connections;
 +------------------------+
 | @@max_user_connections |
 +------------------------+
 |                      0 |
 +------------------------+
 1 row in set (0.00 sec)
 mysql>  set  @@global.max_connections=200;
 Query OK, 0 rows affected (0.00 sec)
 mysql> select @@max_connections;
 +-------------------+
 | @@max_connections |
 +-------------------+
 |               200 |
 +-------------------+
 1 row in set (0.00 sec)


结论：

max_user_connections：限制每个用户的session连接个数，例如max_user_connections=1 ，那么用户u1只能连接的session数为1，如果还有用户u2，还是可以连接，但是连接数仍然为1
max_connections ：是对整个服务器的用户限制，整个服务器只能开这么多session，而不考虑用户！