----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！
