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。