一条统计Group By语句优化

在检查慢SQL时，发现一条统计SQL执行过慢，如下：

原SQL

SELECT platform, channel, COUNT(DISTINCT(platformUserId)) as cnt FROM(  SELECT platform, channel, platformUserId, MIN(insertTimestamp) as rtime  FROM tsz_user    GROUP BY platform, channel, platformUserId  ) a where a.rtime >= 1392393600 and a.rtime < 1392480000  GROUP BY platform, channel;

执行时间：

耗时2分33秒

优化后SQL

SELECT platform, channel, COUNT(DISTINCT(platformUserId)) as cnt FROM(  SELECT platform, channel, platformUserId, MIN(insertTimestamp) as rtime  FROM tsz_user    GROUP BY platform, channel, platformUserId order by null  ) a where a.rtime >= 1392393600 and a.rtime < 1392480000  GROUP BY platform, channel order by null;

执行时间：

耗时55.22秒

执行计划：

结论：

默认情况下，MySQL对所有GROUP BY col1，col2...的字段进行排序。如果查询包括GROUP BY，想要避免排序结果的消耗，则可以指定ORDER By NULL禁止排序。

参考手册：