MySQL Explain详解

核心提示： 派生表的select(FROM子句的子查询)mysql>explainselect*from(select*fromt3whereid=3952602)a;++-+++---+-+-+--+--+---+|id|select_type|table|type|possible_keys

派生表的select(FROM子句的子查询)　

mysql>explainselect*from(select*fromt3whereid=3952602)a;　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+　
　　|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+　
　　|1|PRIMARY||system|NULL|NULL|NULL|NULL|1||　
　　|2|DERIVED|t3|const|PRIMARY,idx_t3_id|PRIMARY|4||1||　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+

3.table

显示这一行的数据是关于哪张表的.

有时不是真实的表名字,看到的是derivedx(x是个数字,我的理解是第几步执行的结果)　

mysql>explainselect*from(select*from(select*fromt3whereid=3952602)a)b;　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+　
　　|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+　
　　|1|PRIMARY||system|NULL|NULL|NULL|NULL|1||　
　　|2|DERIVED||system|NULL|NULL|NULL|NULL|1||　
　　|3|DERIVED|t3|const|PRIMARY,idx_t3_id|PRIMARY|4||1||　
　　+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+