Django-优化数据库查询

##一次检索你需要的所有东西

在不同的位置多次访问数据库，一次获取一个数据集，通常来说不如在一次查询中获取它们更高效。如果你在一个循环中执行查询，这尤其重要。有可能你会做很多次数据库查询，但只需要一次就够了。

我们需要充分了解并使用 select_related() 和 prefetch_related()

###select_related()

Returns a QuerySet that will “follow” foreign-key relationships, selecting additional related-object data when it executes its query. This is a performance booster which results in a single more complex query but means later use of foreign-key relationships won’t require database queries.

The following examples illustrate the difference between plain lookups and select_related() lookups. Here’s standard lookup:

# Hits the database. e = Entry.objects.get(id=5) # Hits the database again to get the related Blog object. b = e.blog

And here’s select_related lookup:

# Hits the database. e = Entry.objects.select_related('blog').get(id=5) # Doesn't hit the database, because e.blog has been prepopulated # in the previous query. b = e.blog

You can use select_related() with any queryset of objects:

from django.utils import timezone # Find all the blogs with entries scheduled to be published in the future. blogs = set() for e in Entry.objects.filter(pub_date__gt=timezone.now()).select_related('blog'): # Without select_related(), this would make a database query for each # loop iteration in order to fetch the related blog for each entry. blogs.add(e.blog)

The order of filter() and select_related() chaining isn’t important. These querysets are equivalent:

Entry.objects.filter(pub_date__gt=timezone.now()).select_related('blog') Entry.objects.select_related('blog').filter(pub_date__gt=timezone.now())

You can follow foreign keys in a similar way to querying them. If you have the following models:

from django.db import models class City(models.Model): # ... pass class Person(models.Model): # ... hometown = models.ForeignKey(City) class Book(models.Model): # ... author = models.ForeignKey(Person)

… then a call to Book.objects.select_related('author__hometown').get(id=4) will cache the related Person and the related City:

b = Book.objects.select_related('author__hometown').get(id=4) p = b.author # Doesn't hit the database. c = p.hometown # Doesn't hit the database. b = Book.objects.get(id=4) # No select_related() in this example. p = b.author # Hits the database. c = p.hometown # Hits the database.

###prefetch_related()

待补充

##不要查询你不需要的东西

###使用 QuerySet.values() 和 values_list()
当你仅仅想要一个带有值的字典或者列表，并不需要使用ORM模型对象时，可以适当使用 values() 。 ###使用 QuerySet.count()
如果你想要获取大小，不要使用 len(queryset) ###使用 QuerySet.exists()
如果你想要知道是否存在至少一个结果，不要使用 if queryset ###使用 QuerySet.update() 和 delete()
通过 QuerySet.update() 使用批量的SQL UPDATE语句，而不是获取大量对象，设置一些值再单独保存。与此相似，在可能的地方使用批量 deletes 。

但是要注意，这些批量的更新方法不会在单独的实例上面调用 save() 或者 delete() 方法，意思是任何你向这些方法添加的自定义行为都不会被执行，包括由普通数据库对象的信号驱动的任何方法。

###直接使用外键的值

如果你仅仅需要外键当中的一个值，要使用对象上你已经取得的外键的值，而不是获取整个关联对象再得到它的主键。例如，执行：

entry.blog_id

而不是：

entry.blog.id