一条统计Group By语句优化

更新时间：2022-09-09 08:51:37

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：

本文转自hcymysql51CTO博客，原文链接： http://blog.51cto.com/hcymysql/1359738，如需转载请自行联系原作者

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