联接的Django订购效率-我可以强制执行子查询吗？

Question

This concerns Django 1.4.6 with a MySQL backend. When doing an ordered query with joins, Django will put the ordering clause at the end of the query. Imagine some models like

class MainObject(models.Model):
    pass

class RelatedObject(models.Model):
    main_object = models.ForeignKey(MainObject)

If you have a lot of these and try a query such as RelatedObject.objects.select_related('main_object').order_by('-id') limit 20000 it is, depending on how many joins there are, almost twice as slow as the same query without an ordering: RelatedObject.objects.select_related('main_object') limit 20000

This is because the query generated is

SELECT app_relatedobject.id, app_relatedobject.main_object_id, app_mainobject.id 
FROM app_relatedobject 
INNER JOIN app_mainobject ON (app_relatedobject.main_object_id = app_mainobject.id) 
ORDER BY app_relatedobject.id DESC
LIMIT 20000

which is much slower than the equivalent

SELECT tmp.id, tmp.main_object_id, app_mainobject.id 
FROM (
    SELECT app_relatedobject.id, app_relatedobject.main_object_id 
    FROM app_relatedobject 
    ORDER BY app_relatedobject.id DESC
    LIMIT 20000) tmp 
INNER JOIN app_mainobject ON (app_relatedobject.main_object_id = tmp.id)

Is there any way to do what I'm trying to achieve in an efficient way? For context, I'm using the django admin system and have a list_display that requires lots of joins across thousands of large records. I can't use raw sql because the rest of the admin system requires a queryset for pagination and filtering.

Any help is greatly appreciated.

Answer 1

You can use your SQL query directly. Django allows for raw SQL queries out-of-the-box and it does return a QuerySet (specifically a RawQuerySet), which behaves as a normal QuerySet as far as I can tell.