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dependent subquery： 子查询中的第一个select查询，依赖于外部查询的结果集
- mysql> explain select * from test where id in (select id from test where id = 1000 union all select id from test2) ;
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
- | 1 | PRIMARY | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
- | 2 | DEPENDENT SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
- | 3 | DEPENDENT UNION | test2 | eq_ref | PRIMARY | PRIMARY | 8 | func | 1 | Using index |
- | NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
- +----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
derived： 用于from子句中有子查询的情况，mysql会递归执行这些子查询，此结果集放在临时表中
- mysql> explain select * from (select * from test2 where id = 1000)a;
- +----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
- | 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | NULL |
- | 2 | DERIVED | test2 | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
- +----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
- 3、table
3、table
table用来表示输出行所引用的表名
4、type(重要)
type表示访问类型，下面依次解释各种类型，类型顺序从最好到最差排列 （编辑：核心网）
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