這是本系列的最後一篇，主要是select_related() 和 prefetch_related() 的最佳實踐。
第一篇在這裡 講例子和select_related()
第二篇在這裡 講prefetch_related()
4. 一些例項
選擇哪個函式
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：
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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()) ... |
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法，讓我們來看看。
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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()) ... |
因為是一個深度為2的prefetch，所以會導致3次SQL查詢：
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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); |
嗯…看上去不錯，但是3次查詢麼？倒過來查詢可能會更簡單？
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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 | 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()的地方儘量使用它，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式。
在我們一直使用的例子上加一個model：Order （訂單）
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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 |
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？
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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 ... |
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
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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 | 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) |
更好的辦法是先呼叫一次select_related()再呼叫prefetch_related()，最後再select_related()後面的表
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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 ... |
這樣只會有3次SQL查詢，Django會先做select_related，之後prefetch_related的時候會利用之前快取的資料，從而避免了1次額外的SQL查詢：
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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 | 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，可以在呼叫prefetch_related之前呼叫select_related，並且Django會按照你想的去做：先select_related，然後利用快取到的資料prefetch_related。然而一旦prefetch_related已經呼叫，select_related將不起作用。
小結
- 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。
- 鑑於第一條，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()。
- 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。
- 只有prefetch_related()之前的select_related()是有效的，之後的將會被無視掉。
關於這兩個函式，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，我會在這篇博文中新增。