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sql连表group by临时表怎么优化

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sql连表group by临时表怎么优化

image.png
两张表关联分组，tot表数量少，分组字段加了索引。当我以tot.busi_id分组的时候就不会用到临时表，如下图
image.png

两种情况都是tot表为驱动表， tor为被驱动表，不加group by是不会用到临时表的，难道排序字段必须是被驱动表上的才可以吗

还有个问题？为啥会用到busi_id索引


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