利用Django架構中select_related和prefetch_related函數對資料庫查詢最佳化

執行個體的背景說明

假定一個個人資訊系統，需要記錄系統中各個人的故鄉、居住地、以及到過的城市。資料庫設計如下：

Models.py 內容如下：

from django.db import models class Province(models.Model): name = models.CharField(max_length=10) def __unicode__(self):  return self.name class City(models.Model): name = models.CharField(max_length=5) province = models.ForeignKey(Province) def __unicode__(self):  return self.name class Person(models.Model): firstname = models.CharField(max_length=10) lastname = models.CharField(max_length=10) visitation = models.ManyToManyField(City, related_name = "visitor") hometown = models.ForeignKey(City, related_name = "birth") living  = models.ForeignKey(City, related_name = "citizen") def __unicode__(self):  return self.firstname + self.lastname

注1：建立的app名為“QSOptimize”

注2：為了簡化起見，`qsoptimize_province` 表中只有2條資料：湖北省和廣東省，`qsoptimize_city`表中只有三條資料：武漢市、十堰市和廣州市

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

1. 因為select_related()總是在單次SQL查詢中解決問題，而prefetch_related()會對每個相關表進行SQL查詢，因此select_related()的效率通常比後者高。
2. 鑒於第一條，儘可能的用select_related()解決問題。只有在select_related()不能解決問題的時候再去想prefetch_related()。
3. 你可以在一個QuerySet中同時使用select_related()和prefetch_related()，從而減少SQL查詢的次數。
4. 只有prefetch_related()之前的select_related()是有效，之後的將會被無視掉。

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