4.一些实例
如果我们想要获得所有家乡是湖北的人，最无脑的做法是先获得湖北省，再获得湖北的所有城市，最后获得故乡是这个城市的人。就像这样：
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>>> hb = Province.objects.get(name__iexact=u"湖北省")
>>> people = []
>>> for city in hb.city_set.all():
... people.extend(city.birth.all())
... |
显然这不是一个明智的选择，因为这样做会导致1+（湖北省城市数）次SQL查询。反正是个反例，导致的查询和获得掉结果就不列出来了。
prefetch_related() 或许是一个好的解决方法，让我们来看看。
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>>> hb = Province.objects.prefetch_related("city_set__birth").objects.get(name__iexact=u"湖北省")
>>> people = []
>>> for city in hb.city_set.all():
... people.extend(city.birth.all())
... |
因为是一个深度为2的prefetch，所以会导致3次SQL查询：
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SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name`
FROM `QSOptimize_province`
WHERE `QSOptimize_province`.`name` LIKE '湖北省' ;
SELECT `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id`
FROM `QSOptimize_city`
WHERE `QSOptimize_city`.`province_id` IN (1);
SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`,
`QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` FROM `QSOptimize_person`
WHERE `QSOptimize_person`.`hometown_id` IN (1, 3);
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嗯…看上去不错，但是3次查询么？倒过来查询可能会更简单？
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>>> people = list(Person.objects.select_related("hometown__province").filter(hometown__province__name__iexact=u"湖北省"))
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SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`,
`QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id`, `QSOptimize_city`.`id`, `QSOptimize_city`.`name`, `QSOptimize_city`.`province_id`, `QSOptimize_province`.`id`, `QSOptimize_province`.`name`
FROM `QSOptimize_person`
INNER JOIN `QSOptimize_city` ON (`QSOptimize_person`.`hometown_id` = `QSOptimize_city`.`id`)
INNER JOIN `QSOptimize_province` ON (`QSOptimize_city`.`province_id` = `QSOptimize_province`.`id`)
WHERE `QSOptimize_province`.`name` LIKE '湖北省';
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+----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
| id | firstname | lastname | hometown_id | living_id | id | name | province_id | id | name |
+----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
| 1 | 张 | 三 | 3 | 1 | 3 | 十堰市 | 1 | 1 | 湖北省 || 2 | 李 | 四 | 1 | 3 | 1 | 武汉市 | 1 | 1 | 湖北省 || 3 | 王 | 麻子 | 3 | 2 | 3 | 十堰市 | 1 | 1 | 湖北省 |+----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
3 rows in set (0.00 sec)
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完全没问题。不仅SQL查询的数量减少了，python程序上也精简了。
select_related()的效率要高于prefetch_related()。因此，最好在能用select_related()的地方尽量使用它，也就是说，对于ForeignKey字段，避免使用prefetch_related()。
联用
对于同一个QuerySet，你可以同时使用这两个函数。
在我们一直使用的例子上加一个model：Order （订单）
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class Order(models.Model):
customer = models.ForeignKey(Person)
orderinfo = models.CharField(max_length=50)
time = models.DateTimeField(auto_now_add = True)
def __unicode__(self):
return self.orderinfo
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如果我们拿到了一个订单的id 我们要知道这个订单的客户去过的省份。因为有ManyToManyField显然必须要用prefetch_related()。如果只用prefetch_related()会怎样呢？
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>>> plist = Order.objects.prefetch_related('customer__visitation__province').get(id=1)
>>> for city in plist.customer.visitation.all():
... print city.province.name
... |
显然，关系到了4个表：Order、Person、City、Province，根据prefetch_related()的特性就得有4次SQL查询
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SELECT `QSOptimize_order`.`id`, `QSOptimize_order`.`customer_id`, `QSOptimize_order`.`orderinfo`, `QSOptimize_order`.`time`
FROM `QSOptimize_order`
WHERE `QSOptimize_order`.`id` = 1 ;
SELECT `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`, `QSOptimize_person`.`lastname`, `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id`
FROM `QSOptimize_person`
WHERE `QSOptimize_person`.`id` IN (1);
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);
SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name`
FROM `QSOptimize_province`
WHERE `QSOptimize_province`.`id` IN (1, 2);
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+----+-------------+---------------+---------------------+| id | customer_id | orderinfo | time |+----+-------------+---------------+---------------------+| 1 | 1 | Info of Order | 2014-08-10 17:05:48 |+----+-------------+---------------+---------------------+1 row in set (0.00 sec)+----+-----------+----------+-------------+-----------+| id | firstname | lastname | hometown_id | living_id |+----+-----------+----------+-------------+-----------+| 1 | 张 | 三 | 3 | 1 |+----+-----------+----------+-------------+-----------+1 row in set (0.00 sec)+-----------------------+----+--------+-------------+| _prefetch_related_val | id | name | province_id |+-----------------------+----+--------+-------------+| 1 | 1 | 武汉市 | 1 || 1 | 2 | 广州市 | 2 || 1 | 3 | 十堰市 | 1 |+-----------------------+----+--------+-------------+3 rows in set (0.00 sec)+----+--------+| id | name |+----+--------+| 1 | 湖北省 || 2 | 广东省 |+----+--------+2 rows in set (0.00 sec) |
更好的办法是先调用一次select_related()再调用prefetch_related()，最后再select_related()后面的表
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>>> plist = Order.objects.select_related('customer').prefetch_related('customer__visitation__province').get(id=1)
>>> for city in plist.customer.visitation.all():
... print city.province.name
... |
这样只会有3次SQL查询，Django会先做select_related，之后prefetch_related的时候会利用之前缓存的数据，从而避免了1次额外的SQL查询：
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SELECT `QSOptimize_order`.`id`, `QSOptimize_order`.`customer_id`, `QSOptimize_order`.`orderinfo`,
`QSOptimize_order`.`time`, `QSOptimize_person`.`id`, `QSOptimize_person`.`firstname`,
`QSOptimize_person`.`lastname`, `QSOptimize_person`.`hometown_id`, `QSOptimize_person`.`living_id` FROM `QSOptimize_order`
INNER JOIN `QSOptimize_person` ON (`QSOptimize_order`.`customer_id` = `QSOptimize_person`.`id`)
WHERE `QSOptimize_order`.`id` = 1 ;
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);
SELECT `QSOptimize_province`.`id`, `QSOptimize_province`.`name`
FROM `QSOptimize_province`
WHERE `QSOptimize_province`.`id` IN (1, 2);
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+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+| id | customer_id | orderinfo | time | id | firstname | lastname | hometown_id | living_id |+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+| 1 | 1 | Info of Order | 2014-08-10 17:05:48 | 1 | 张 | 三 | 3 | 1 |+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+1 row in set (0.00 sec)+-----------------------+----+--------+-------------+| _prefetch_related_val | id | name | province_id |+-----------------------+----+--------+-------------+| 1 | 1 | 武汉市 | 1 || 1 | 2 | 广州市 | 2 || 1 | 3 | 十堰市 | 1 |+-----------------------+----+--------+-------------+3 rows in set (0.00 sec)+----+--------+| id | name |+----+--------+| 1 | 湖北省 || 2 | 广东省 |+----+--------+2 rows in set (0.00 sec) |
值得注意的是，可以在调用prefetch_related之前调用select_related，并且Django会按照你想的去做：先select_related，然后利用缓存到的数据prefetch_related。然而一旦prefetch_related已经调用，select_related将不起作用。
小结
- 因为select_related()总是在单次SQL查询中解决问题，而prefetch_related()会对每个相关表进行SQL查询，因此select_related()的效率通常比后者高。
- 鉴于第一条，尽可能的用select_related()解决问题。只有在select_related()不能解决问题的时候再去想prefetch_related()。
- 你可以在一个QuerySet中同时使用select_related()和prefetch_related()，从而减少SQL查询的次数。
- 只有prefetch_related()之前的select_related()是有效的，之后的将会被无视掉。