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：






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