MySQL何時將對使用order by子句的查詢應用索引條件推送？為什么？

Question

我有一個帶有多列索引（author_id，reply_time，id）的文章表。

explain select * from article where author_id=3658768 and reply_time>'2015-01-01' and id>85669107 order by reply_time
explain select * from article where author_id=3658768 and reply_time>'2015-01-01' and id<85669107 order by reply_time
explain select * from article where author_id=3658768 and reply_time<'2015-01-01' and id>85669107 order by reply_time
explain select * from article where author_id=3658768 and reply_time<'2015-01-01' and id<85669107 order by reply_time

將使用索引條件推送（說明輸出的“額外”字段為“使用索引條件”）

但是，以下所有查詢將不使用索引條件推送（explain輸出的“ extra”字段為“ Using where”）

explain select * from article where author_id=3658768 and reply_time>'2015-01-01' and id>85669107 order by reply_time desc
explain select * from article where author_id=3658768 and reply_time>'2015-01-01' and id<85669107 order by reply_time desc
explain select * from article where author_id=3658768 and reply_time<'2015-01-01' and id>85669107 order by reply_time desc
explain select * from article where author_id=3658768 and reply_time<'2015-01-01' and id<85669107 order by reply_time desc

MySQL是否僅按索引順序使用索引條件推送？

Answer 1

通過author_id, reply_time, id進行索引幾乎沒有意義author_id, reply_time, id除非您確定同一author_id, reply_time會有多個id 。 索引不會比僅通過author_id, reply_time僅author_id, reply_time更好。

您最好使用兩個索引，一個通過author_id, reply_time和另一個通過author_id, id索引，並且計划者將使用最有可能（從收集的統計數據中）得出的索引數量將減少的行數。按順序過濾。

不管排序是升序還是降序都是無關緊要的，計划者可以利用索引進行排序，也可以很好地使用它進行升序或降序。 但是有時它根本無法使用它，或者使用它對行集進行排序最昂貴。