这是本系列的最后一篇,主要是select_related() 和 prefetch_related() 的最佳实践。
4. 一些实例
选择哪个函数
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:
- >>> 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。
- >>> 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:
- 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);
嗯...看上去不错,但是3次查询么?倒过来查询可能会更简单?
- >>> people = list(Person.objects.select_related("hometown__province").filter(hometown__province__name__iexact=u"湖北省"))
- 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';
- +----+-----------+----------+-------------+-----------+----+--------+-------------+----+--------+
- | 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)
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)
- 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
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?
- >>> 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
- 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);
- +----+-------------+---------------+---------------------+
- | 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
- >>> 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:
- 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);
- +----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+
- | 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。