MySQL Explain详解

核心提示： (2). PRIMARY我的理解是最外层的select.例如:mysql>explainselect*from(select*fromt3whereid=3952602)a;++-+++---+-+-+--+--+---+|id|select_type|table|type|poss

(2). PRIMARY

我的理解是最外层的select.例如:

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).union

union中的第二个或后面的select语句.例如

mysql>explainselect*fromt3whereid=3952602unionallselect*fromt3;　
　　+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+　
　　|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|　
　　+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+　
　　|1|PRIMARY|t3|const|PRIMARY,idx_t3_id|PRIMARY|4|const|1||　
　　|2|union|t3|ALL|NULL|NULL|NULL|NULL|1000||　
　　|NULL|unionRESULT||ALL|NULL|NULL|NULL|NULL|NULL||　
　　+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+