慎用mysql嵌套

昨天弄了句SQL，把数据库给弄crash了，引以为戒
差不多是这个样子的

SELECT Photo.* FROM Photo WHERE (Photo.Parent in (select CONCAT('AssociationEvent-', AssociationEvent.ID) as Parent from AssociationEvent WHERE AssociationEvent.Association = 98 union select CONCAT('PersonAssociation-', PersonAssociation.ID) as Parent from PersonAssociation WHERE PersonAssociation.IsBrothersOnly = 0 and PersonAssociation.Association = 98)) ORDER BY Sequence DESC  LIMIT 8 

这个in里面有个嵌套語句

mysql> explain SELECT Photo.* FROM Photo WHERE (Photo.Parent in (select CONCAT('AssociationEvent-', AssociationEvent.ID) as Parent from AssociationEvent WHERE AssociationEvent.Association = 98 union select CONCAT('PersonAssociation-', PersonAssociation.ID) as Parent from PersonAssociation WHERE PersonAssociation.IsBrothersOnly = 0 and PersonAssociation.Association = 98)) ORDER BY Sequence DESC  LIMIT 8 ;
+----+--------------------+-------------------+--------+---------------+----------+---------+------+------+-------------+
| id | select_type        | table             | type   | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+--------------------+-------------------+--------+---------------+----------+---------+------+------+-------------+
|  1 | PRIMARY            | Photo             | index  | NULL          | Sequence | 5       | NULL | 3199 | Using where | 
|  2 | DEPENDENT SUBQUERY | AssociationEvent  | system | NULL          | NULL     | NULL    | NULL |    1 |             | 
|  3 | DEPENDENT UNION    | PersonAssociation | system | NULL          | NULL     | NULL    | NULL |    1 |             | 
| NULL | UNION RESULT       | <union2,3>        | ALL    | NULL          | NULL     | NULL    | NULL | NULL |             | 
+----+--------------------+-------------------+--------+---------------+----------+---------+------+------+-------------+
4 rows in set (0.00 sec)

可以看到，mysql执行这句话时，会有很多步骤，当数据量一大，它就不行了