本文主要向大家介绍了MySQL数据库之记一次order by desc limit导致的查询慢 ，通过具体的内容向大家展现，希望对大家学习MySQL数据库有所帮助。

    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~~

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