有一次去online set wait_timeout 的时候发现改了不生效，如下：
mysql> show variables like ‘wait_timeout‘;
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout | 100 |
+---------------+-------+
1 row in set (0.00 sec)
mysql> set global wait_timeout=28800;
Query OK, 0 rows affected (0.00 sec)
mysql> exit
Bye
退出后重新登录mysql
mysql> show variables like ‘wait_timeout‘;
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout | 100 |
+---------------+-------+
1 row in set (0.00 sec)
网上搜索了一下：
说法1：interactive_timeout和wait_timeout的默认值都是28800（8小时）当这两个参数同时出现在里时，会以interactive_timeout的值为准。也就是说不管wait_timeout的值是多少，用show variables like ‘%timeout%‘;查看时显示的两个值都是一样的，并且都是interactive_timeout的值。
说法2：如果查询时使用的是show variables的话，会发现设置好像并没有生效，这是因为单纯使用show variables的话就等同于使用的是show session variables，查询的是会话变量，只有使用show global variables，查询的才是全局变量。网络上很多人都抱怨说他们set global之后使用show variables查询没有发现改变，原因就在于混淆了会话变量和全局变量，如果仅仅想修改会话变量的话，可以使用类似set wait_timeout=10;或者set session wait_timeout=10;这样的语法。
验证一下说法1：修改interactive_timeout 是否可以达到修改wait_timeout的效果
mysql> show variables like ‘%timeout‘;
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| interactive_timeout | 200 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| table_lock_wait_timeout | 50 |
| wait_timeout | 200 |
+-------------------------+-------+
8 rows in set (0.00 sec)
mysql> set global interactive_timeout=100;
Query OK, 0 rows affected (0.00 sec)
mysql>show variables like ‘%timeout‘;
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| interactive_timeout | 200 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| table_lock_wait_timeout | 50 |
| wait_timeout | 200 |
+-------------------------+-------+
8 rows in set (0.00 sec)
mysql> show global variables like ‘%timeout‘;
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| interactive_timeout | 100 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| table_lock_wait_timeout | 50 |
| wait_timeout | 200 |
+-------------------------+-------+
8 rows in set (0.00 sec)
mysql> exit
Bye
退出后重新登录mysql
mysql> show variables like ‘%timeout‘;
+-------------------------+-------+
| Variable_name | Value |
+-------------------------+-------+
| connect_timeout | 10 |
| delayed_insert_timeout | 300 |
| interactive_timeout | 100 |
| net_read_timeout | 30 |
| net_write_timeout | 60 |
| slave_net_timeout | 3600 |
| table_lock_wait_timeout | 50 |
| wait_timeout | 100 |
+-------------------------+-------+
8 rows in set (0.00 sec)
mysql>
以上可以看到，如果修改interactive_timeout的话wait_timeout也会跟着变，而只修改wait_timeout是不生效的。
