MySQL Explain详解

核心提示： (4).DEPENDENT unionunion中的第二个或后面的select语句，取决于外面的查询mysql>explainselect*fromt3whereidin(selectidfromt3whereid=3952602unionallselectidfromt3);+--

(4).DEPENDENT union

union中的第二个或后面的select语句，取决于外面的查询

mysql>explainselect*fromt3whereidin(selectidfromt3whereid=3952602unionallselectidfromt3);　
　　+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+　
　　|id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra|　
　　+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+　
　　|1|PRIMARY|t3|ALL|NULL|NULL|NULL|NULL|1000|Usingwhere|　
　　|2|DEPENDENTSUBQUERY|t3|const|PRIMARY,idx_t3_id|PRIMARY|4|const|1|Usingindex|　
　　|3|DEPENDENTunion|t3|eq_ref|PRIMARY,idx_t3_id|PRIMARY|4|func|1|Usingwhere;Usingindex|　
　　|NULL|unionRESULT||ALL|NULL|NULL|NULL|NULL|NULL||　
　　+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+--------------------------+

(5).union RESULT

union的结果。

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||　
　　+----+--------------+------------+-------+-------------------+---------+---------+-------+------+-------+