子查询形式的执行计划

mysql> explain select * from user where id = (select uid from uu where id = 3)\G *************************** 1. row ***************************            id: 1   select_type: PRIMARY table: user          type: const possible_keys: PRIMARY key: PRIMARY       key_len: 4           ref: const          rows: 1         Extra: *************************** 2. row ***************************            id: 2   select_type: SUBQUERY table: uu          type: const possible_keys: PRIMARY key: PRIMARY       key_len: 4           ref:          rows: 1         Extra: 2 rows in set (0.00 sec)

子查询的结果

mysql> select * from user where id = (select uid from uu where id = 3); +----+------+------+ | id | name | age  | +----+------+------+ |  9 | qw13 |   18 | +----+------+------+

连接查询的执行计划

mysql> explain select * from user left join uu on user.id = uu.uid where uu.id=3  \G *************************** 1. row ***************************            id: 1   select_type: SIMPLE table: uu          type: const possible_keys: PRIMARY,uid key: PRIMARY       key_len: 4           ref: const          rows: 1         Extra: *************************** 2. row ***************************            id: 1   select_type: SIMPLE table: user          type: const possible_keys: PRIMARY key: PRIMARY       key_len: 4           ref: const          rows: 1         Extra: 2 rows in set (0.02 sec)

链接查询的结果

mysql> select * from user left join uu on user.id = uu.uid where uu.id=3; +----+------+------+----+------+-------+ | id | name | age  | id | uid  | other | +----+------+------+----+------+-------+ |  9 | qw13 |   18 |  3 |    9 | asd   | +----+------+------+----+------+-------+

未完待续……