记一次order by desc limit导致的查询慢

Arce

　　昨天接到一个客户的问题，电脑上可以打开网站，在手机上确不能打开报500的错。首先登陆上客户的服务器查看环境apache+mysql+php，php和mysql的占用都比较高，按经验来说那就是mysql的问题了，登陆mysql用show processlist查看进程，发现一条查询一直在sending date

mysql> show processlist;
+------+------+-----------------+--------+---------+------+--------------+------
--------------------------------------------------------------------------------
----------------+
| Id   | User | Host      | db   | Command | Time | State    | Info
        |
+------+------+-----------------+--------+---------+------+--------------+------
--------------------------------------------------------------------------------
----------------+
| 1832 | root | localhost:53490 | NULL   | Query   |  0 | NULL     | show
processlist
        |
| 1842 | root | localhost:53508 | yungou | Query   |  4 | Sending data | selec
t a.id,a.q_user,a.q_showtime,a.thumb,a.title,a.q_uid,qishu,announced_type,q_end_
time ,(SELECT ` |
+------+------+-----------------+--------+---------+------+--------------+------
--------------------------------------------------------------------------------
----------------+
2 rows in set (0.00 sec)

　　明显不对啊，把这条语句单独拿出来执行也是慢得要死48s。
　　explain分析一下这条语句：

mysql> explain select a.id,a.q_user,a.q_showtime,a.thumb,a.title,a.q_uid,qishu,a
nnounced_type,q_end_time ,(SELECT `time` FROM `go_member_go_record` WHERE shopid
 = a.id ORDER BY `time` DESC LIMIT 1 ) as gm_time from `go_shoplist` as a where
`shenyurenshu` <=0 ORDER BY `gm_time` DESC LIMIT 4\G;
*************************** 1. row ***************************
       id: 1
  select_type: PRIMARY
    table: a
     type: range
possible_keys: shenyurenshu
      key: shenyurenshu
    key_len: 4
      ref: NULL
     rows: 945
    Extra: Using where; Using filesort
*************************** 2. row ***************************
       id: 2
  select_type: DEPENDENT SUBQUERY
    table: go_member_go_record
     type: index
possible_keys: shopid
      key: time
    key_len: 63
      ref: NULL
     rows: 1
    Extra: Using where
2 rows in set (0.00 sec)

　　一看type是range就悲剧了，慢是肯定的了，但是945条记录也不至于这么慢啊！！

　　没办法用简化法来定位错误，先去掉子查询，直接

select a.id,a.q_user,a.q_showtime,a.thumb,a.title,a.q_uid,qishu,announced_type,q_end_time from `go_shoplist` as a where `shenyurenshu` <=0 ORDER BY `gm_time` DESC LIMIT 4;

　　没有问题，速度杠杠的~~
　　那就是子查询的问题了哦，从哪里动刀呢？order by desc limit
　　首先去掉排序，查询杠杠的~~
　　然后想想order by desc limit 1怎么替代呢，这句的意思是查询最大的值，那我直接用max可以吗？马上修改

select a.id,a.q_user,a.q_showtime,a.thumb,a.title,a.q_uid,qishu,announced_type,q_end_time ,(SELECT MAX(`time`) FROM `go_member_go_record` WHERE shopid = a.id ) as gm_time from `go_shoplist` as a where `shenyurenshu` <=0 ORDER BY `gm_time` DESC LIMIT 4;

　　打开网站，哈哈，速度杠杠的~~