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转 实例详解Django的 select_related 和 prefetch_related 函数对 QuerySet 查询的优化(三)

时间:2019-06-04 19:16:53      阅读:96      评论:0      收藏:0      [点我收藏+]

标签:注意   python程序   精简   原创文章   ext   广州市   order   最佳实践   key   

这是本系列的最后一篇,主要是select_related() 和 prefetch_related() 的最佳实践。

第一篇在这里 讲例子和select_related()

第二篇在这里 讲prefetch_related()

 

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。

 

关于这两个函数,我能想到的东西目前只有这么多。不过基于一些个人原因,写第三篇时间比较短,写的有些仓促。如果什么时候又想起了什么,我会在这篇博文中添加。
---------------------
作者:CuGBabyBeaR
来源:CSDN
原文:https://blog.csdn.net/cugbabybear/article/details/38460877
版权声明:本文为博主原创文章,转载请附上博文链接!

转 实例详解Django的 select_related 和 prefetch_related 函数对 QuerySet 查询的优化(三)

标签:注意   python程序   精简   原创文章   ext   广州市   order   最佳实践   key   

原文地址:https://www.cnblogs.com/saolv/p/10975474.html

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