本文共 642 字，大约阅读时间需要 2 分钟。

Using filesort

排序的时候无法用到索引

EXPLAIN SELECT DISTINCT	hostSn,	engineer_id,	COUNT(*) AS countFROM	wx_repair_orderGROUP BY	engineer_id

Using temporary

使用了临时表：A temporary table is created to hold the result. This typically happens if you are using GROUP BY, DISTINCT or ORDER BY.

EXPLAIN SELECT DISTINCT	hostSn,	engineer_id,	COUNT(*) AS countFROM	wx_repair_orderGROUP BY	engineer_idORDER BY NULL

DERIVED

派生表：实际上是一种特殊的subquery，它位于SQL语句中FROM子句里面，可以看做是一个单独的表。

先去重然后统计hostSn出现的次数。

EXPLAIN SELECT	hostSn,	engineer_id,	COUNT(*) AS countFROM	(		SELECT DISTINCT			hostSn,			engineer_id		FROM			wx_repair_order	) aGROUP BY	engineer_idORDER BY	count DESC

参考:

转载地址：http://pgrai.baihongyu.com/