例項詳解Django的 select_related 和 prefetch_related 函式對 QuerySet 查詢的優化(三)
阿新 • • 發佈:2019-01-24
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: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。>>> hb = Province.objects.get(name__iexact=u"湖北省") >>> people = [] >>> for city in hb.city_set.all(): ... people.extend(city.birth.all()) ...
prefetch_related() 或許是一個好的解決方法,讓我們來看看。
因為是一個深度為2的prefetch,所以會導致3次SQL查詢:>>> 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()) ...
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?
>>> 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)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)
更好的辦法是先呼叫一次select_related()再呼叫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的時候會利用之前快取的資料,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。