在检查慢sql时，发现一条统计sql执行过慢，如下：原sqlselectplatform,channel,count(distinct(platformuserid))ascntfrom(selectplatform,channel,platformuser
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
 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
 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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