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Django ORM自带的defer和only方法虽说不像filter, exclude,
order_by这几个方法那么常见，但用起来还是可以优化数据库查询的。今天小编我就带你看看如何使用这两种方法。
Defer方法
Defer方法的用途是查询数据库时跳过指定的字段，比如下面查询时将跳过每篇Entry的headline和body字段。当你不需要在查询结果中使用headline和body字段时，使用defer方法可以防止将headline和body载入内存，从而节省空间。一篇文章可能没啥关系，但文章很多时，节省的内存空间不可忽视。
Entry.objects.defer("headline", "body")
Defer方法可以和filter, exclude方法联用，其顺序无关紧要，如下所示：
Entry.objects.defer("body").filter(rating=5).defer("headline")
如果你要载入所有字段，只需要设置defer(None)
my_queryset.defer(None)
Only方法
Only方法与defer方法作用类时，只不过only方法是指定需要载入的字段。比如下面查询将只会载入body和pub_date。
Entry.objects.only("body", "pub_date")
使用only方法时一定要非常注意它的顺序，它的执行以最后一个为准。下例将只会载入headline。
Entry.objects.only("body", "rating").only("headline")
only方法与defer方法可以联用，但一定要注意先后顺序。下例只会载入headline。
Entry.objects.only("headline", "body").defer("body")
