MySQL中索引优化distinct语句及distinct的多字段操作
MySQL通常使用GROUPBY(本质上是排序动作)完成DISTINCT操作,如果DISTINCT操作和ORDERBY操作组合使用,通常会用到临时表.这样会影响性能.在一些情况下,MySQL可以使用索引优化DISTINCT操作,但需要活学活用.本文涉及一个不能利用索引完成DISTINCT操作的实例.
实例1使用索引优化DISTINCT操作
createtablem11(aint,bint,cint,dint,primarykey(a))engine=INNODB; insertintom11values(1,1,1,1),(2,2,2,2),(3,3,3,3),(4,4,4,4),(5,5,5,5),(6,6,6,6),(7,7,7,7),(8,8,8,8); explainselectdistinct(a)fromm11;
mysql>explainselectdistinct(a)fromm11;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+|id|select_type|table|partitions|type|possible_keys|key|key_len|ref|rows|filtered|Extra|+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+|1|SIMPLE|m11|NULL|index|PRIMARY|PRIMARY|4|NULL|1|100.00|Usingindex|+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+