執行個體詳解Django的 select_related 和 prefetch_related 函數對 QuerySet 查詢的最佳化（三），djangoqueryset

執行個體詳解Django的 select_related 和 prefetch_related 函數對 QuerySet 查詢的最佳化（三），djangoqueryset

這是本系列的最後一篇，主要是select_related() 和 prefetch_related() 的最佳實務。

第一篇在這裡 講例子和select_related()

第二篇在這裡 講prefetch_related()

4. 一些執行個體
選擇哪個函數如果我們想要獲得所有家鄉是湖北的人，最無腦的做法是先獲得湖北省，再獲得湖北的所有城市，最後獲得故鄉是這個城市的人。就像這樣：

>>> hb = Province.objects.get(name__iexact=u"湖北省")>>> people = []>>> for city in hb.city_set.all():...   people.extend(city.birth.all())...

顯然這不是一個明智的選擇，因為這樣做會導致1+（湖北省城市數）次SQL查詢。反正是個反例，導致的查詢和獲得掉結果就不列出來了。
prefetch_related() 或許是一個好的解決方案，讓我們來看看。

>>> 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())...

因為是一個深度為2的prefetch，所以會導致3次SQL查詢：

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...看上去不錯，但是3次查詢嗎？倒過來查詢可能會更簡單？

>>> 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 | 張        | 三       |           3 |         1 |  3 | 十堰市 |           1 |  1 | 湖北省 ||  2 | 李        | 四       |           1 |         3 |  1 | 武漢市 |           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)

完全沒問題。不僅SQL查詢的數量減少了，python程式上也精簡了。
select_related()的效率要高於prefetch_related()。因此，最好在能用select_related()的地方盡量使用它，也就是說，對於ForeignKey欄位，避免使用prefetch_related()。


聯用對於同一個QuerySet，你可以同時使用這兩個函數。在我們一直使用的例子上加一個model：Order （訂單）

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

如果我們拿到了一個訂單的id 我們要知道這個訂單的客戶去過的省份。因為有ManyToManyField顯然必須要用prefetch_related()。如果只用prefetch_related()會怎樣呢？

>>> plist = Order.objects.prefetch_related('customer__visitation__province').get(id=1)>>> for city in plist.customer.visitation.all():...   print city.province.name...

顯然，關係到了4個表：Order、Person、City、Province，根據prefetch_related()的特性就得有4次SQL查詢

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 | 張        | 三       |           3 |         1 |+----+-----------+----------+-------------+-----------+1 row in set (0.00 sec)+-----------------------+----+--------+-------------+| _prefetch_related_val | id | name   | province_id |+-----------------------+----+--------+-------------+|                     1 |  1 | 武漢市 |           1 ||                     1 |  2 | 廣州市 |           2 ||                     1 |  3 | 十堰市 |           1 |+-----------------------+----+--------+-------------+3 rows in set (0.00 sec)+----+--------+| id | name   |+----+--------+|  1 | 湖北省 ||  2 | 廣東省 |+----+--------+2 rows in set (0.00 sec)

更好的辦法是先調用一次select_related()再調用prefetch_related()，最後再select_related()後面的表

>>> 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...

這樣只會有3次SQL查詢，Django會先做select_related，之後prefetch_related的時候會利用之前緩衝的資料，從而避免了1次額外的SQL查詢：

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 | 張        | 三       |           3 |         1 |+----+-------------+---------------+---------------------+----+-----------+----------+-------------+-----------+1 row in set (0.00 sec)+-----------------------+----+--------+-------------+| _prefetch_related_val | id | name   | province_id |+-----------------------+----+--------+-------------+|                     1 |  1 | 武漢市 |           1 ||                     1 |  2 | 廣州市 |           2 ||                     1 |  3 | 十堰市 |           1 |+-----------------------+----+--------+-------------+3 rows in set (0.00 sec)+----+--------+| id | name   |+----+--------+|  1 | 湖北省 ||  2 | 廣東省 |+----+--------+2 rows in set (0.00 sec)

值得注意的是，可以在調用prefetch_related之前調用select_related，並且Django會按照你想的去做：先select_related，然後利用緩衝到的資料prefetch_related。然而一旦prefetch_related已經調用，select_related將不起作用。

小結


關於這兩個函數，我能想到的東西目前只有這麼多。不過基於一些個人原因，寫第三篇時間比較短，寫的有些倉促。如果什麼時候又想起了什麼，我會在這篇博文中添加。

問一個django中queryset的問題

from itertools import chainfrom operator import attrgetter#...post = Post.objects.get(pk=post_id) # 擷取博文likes = likes = post.like_set.all() # 擷取喜愛資訊# likes = Like.objects.filter(post=post)reblogs = Post.objects.filter(reblog_from=post) # 擷取轉寄資訊# 合并喜愛及轉寄資訊，並按時間逆序排序notes = sorted(chain(likes, reblogs), key=attrgetter('created_at'), reverse=True)#...使用 itertools.chain 函數合并可迭代對象，查詢集為可迭代對象：
>>>list(chain([1,2,3],'abc'))>>>[1,2,3,'a','b','c']使用 sorted 函數排序（按對象屬性排序）。

 
DJANGO問題，queryset的delete方法

你試試下面這中寫法，看是不是會全部刪除
> from models import Entry
> query = Entry.all()
> entries =query.fetch(1)
> db.delete(entries)