1. explain select * from operate_log where operate_time<'2019-09-02 16:11:52' and operate_time>'2019-09-02 16:01:52'
group by relate_id order by relate_id desc , operate_time desc
2. explain select * from iwork_operate_log where operate_time<'2019-09-02 16:11:52' and operate_time>'2019-09-02 16:01:52' and relate_id=20 group by relate_id order by relate_id desc , operate_time desc
3. explain select * from iwork_operate_log where operate_time<'2019-09-02 16:11:52' and operate_time>'2019-09-02 16:01:52' group by relate_id
不太理解爲什麼沒有file sort,group by不是也有默認排序嗎,因爲relate_id 本身是索引,本身有序
4. explain select * from iwork_operate_log where operate_time<'2019-09-02 16:11:52' and operate_time>'2019-09-02 16:01:52' group by operate_time
5.groupby取得默認不是Id最小的那個,而是groupby的那個字段的降序的那一條,因此如果order by你groupby的那個字段,可以達到組內排序的效果,其他則不行。