今天论坛打开了这个会话功能，结果很荣幸踩坑里了，连接数直接给干到2000开外。

好了，直接上图说下：

show processlist，满屏显示这条SQL，情急之下，立即pt-kill，先让论坛活下来。顺便想问候下discuz开发，你们真的做了测试了吗，就直接发布了？

表结构：

mysql> desc pre_common_session;
+--------------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------------+----------------------+------+-----+---------+-------+
| sid | char(6) | NO | PRI | | |
| ip1 | tinyint(3) unsigned | NO | | 0 | |
| ip2 | tinyint(3) unsigned | NO | | 0 | |
| ip3 | tinyint(3) unsigned | NO | | 0 | |
| ip4 | tinyint(3) unsigned | NO | | 0 | |
| uid | int(11) unsigned | NO | MUL | 0 | |
| username | char(50) | NO | | | |
| groupid | smallint(6) unsigned | NO | | 0 | |
| invisible | tinyint(1) | NO | | 0 | |
| action | tinyint(1) unsigned | NO | | 0 | |
| lastactivity | int(10) unsigned | NO | | 0 | |
| lastolupdate | int(10) unsigned | NO | | 0 | |
| fid | int(11) unsigned | NO | | 0 | |
| tid | int(11) unsigned | NO | | 0 | |
+--------------+----------------------+------+-----+---------+-------+
14 rows in set (0.00 sec)

由于该SQL导致全表扫描，虽然表数据不多1万多行，但InnoDB引擎在没有使用到索引的情况下，会转为表级锁，并发一大，线程处理不完，连接不释放，连接数就会上升。

最终让开发在后台关闭该功能，解决之。