今天論壇打開了這個會話功能，結果很榮幸踩坑裏了，連接數直接給幹到2000開外。
好了，直接上圖說下：
show processlist，滿屏顯示這條SQL，情急之下，立即pt-kill，先讓論壇活下來。順便想問候下discuz開發，你們真的做了測試了嗎，就直接發佈了？
表結構：
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)
由於該SQL導致全表掃描，雖然表數據不多1萬多行，但InnoDB引擎在沒有使用到索引的情況下，會轉爲表級鎖，併發一大，線程處理不完，連接不釋放，連接數就會上升。
最終讓開發在後臺關閉該功能，解決之。