实例详解Django的select_related和prefetch_related函数对QuerySet查询的优化（二）(二)

SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, 
`QSOptimize_person`.`lastname`, `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` 
FROM `QSOptimize_person` 
WHERE `QSOptimize_person`.`firstname` LIKE '张' ;

SELECT (`QSOptimize_person_visitation`.`person_id`) AS `_prefetch_related_val`, `QSOptimize_city`.`id`,
`QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` FROM `QSOptimize_city` 
INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`)
WHERE `QSOptimize_person_visitation`.`person_id` IN (1, 4);

SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name` 
FROM `QSOptimize_province` 
WHERE `QSOptimize_province`.`id` IN (1, 2);

+----+-----------+----------+-------------+-----------+
| id | firstname | lastname | hometown_id | living_id |
+----+-----------+----------+-------------+-----------+
|  1 | 张        | 三       |           3 |         1 |
|  4 | 张        | 六       |           2 |         2 |
+----+-----------+----------+-------------+-----------+
2 rows in set (0.00 sec)

+-----------------------+----+-----------+-------------+
| _prefetch_related_val | id | name      | province_id |
+-----------------------+----+-----------+-------------+
|                     1 |  1 | 武汉市    |           1 |
|                     1 |  2 | 广州市    |           2 |
|                     4 |  2 | 广州市    |           2 |
|                     1 |  3 | 十堰市    |           1 |
+-----------------------+----+-----------+-------------+
4 rows in set (0.00 sec)

+----+-----------+
| id | name      |
+----+-----------+
|  1 | 湖北省    |
|  2 | 广东省    |
+----+-----------+
2 rows in set (0.00 sec)

值得一提的是，链式prefetch_related会将这些查询添加起来，就像1.7中的select_related那样。

要注意的是，在使用QuerySet的时候，一旦在链式操作中改变了数据库请求，之前用prefetch_related缓存的数据将会被忽略掉。这会导致Django重新请求数据库来获得相应的数据，从而造成性能问题。这里提到的改变数据库请求指各种filter()、exclude()等等最终会改变SQL代码的操作。而all()并不会改变最终的数据库请求，因此是不会导致重新请求数据库的。

举个例子，要获取所有人访问过的城市中带有“市”字的城市，这样做会导致大量的SQL查询：

plist = Person.objects.prefetch_related('visitation')
[p.visitation.filter(name__icontains=u"市") for p in plist]

SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`, 
`QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` 
FROM `QSOptimize_person`;

SELECT (`QSOptimize_person_visitation`.`person_id`) AS `_prefetch_related_val`, `QSOptimize_city`.`id`,
`QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` 
FROM `QSOptimize_city` 
INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`)
WHERE `QSOptimize_person_visitation`.`person_id` IN (1, 2, 3, 4);

SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` 
FROM `QSOptimize_city` 
INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`) 
WHERE(`QSOptimize_person_visitation`.`person_id` = 1  AND `QSOptimize_city`.`name` LIKE '%市%' );

SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id` 
FROM `QSOptimize_city` 
INNER JOIN `QSOptimize_person_visitation` ON (`QSOptimize_city`.`id` = `QSOptimize_person_visitation`.`city_id`) 
WHERE (`QSOptimize_person_visitation`.`person_id` = 2  AND `QSOptimize_city`.`na