Django的 select_related 和 prefetch_related 函数对 QuerySet 查询的优化(三)
4.一些实例
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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 ())
...
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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 ())
...
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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);
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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 | 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)
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联用
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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
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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
...
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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 | 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)
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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
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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
...
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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:
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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 | 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)
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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。
转载于:https://www.cnblogs.com/Edifier-7/p/5381639.html
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