prefetch_one_level is slow

[Simon Percivall]

prefetch_one_level in django.db.models.query.py does a lot of I think unnecessary extra work.

For every obj in instances a QuerySet is created for the related many to many by doing getattr(obj, attname).all(). This QuerySet however is never really used, instead its _prefetched_objects_cache is filled with the prefetched items.

qs = getattr(obj, attname).all()
qs._result_cache = vals
obj._prefetched_objects_cache[cache_name] = qs

If instead you'd simply create a QuerySet you save having to call clone for every obj, which is very expensive.

qs = QuerySet(getattr(obj, attname).model)
qs._result_cache = vals
obj._prefetched_objects_cache[cache_name] = qs

For a function like prefetch_related whose main purpose is to speed up a certain case of use, speed seems like a good quality. Maybe I've misunderstood an edge case here, but if not, this is an easy optimization that gives quite the speedup.

All modeltests on 1.4 pass with this change, by the way.

[Luke Plant]

The problem is that the QuerySet must still behave the same way as the original QuerySet, apart from having its _result_cache already filled. For example, if the user does some_obj.related_objects.all().filter(bar=foo) or even some_obj.related_objects.filter(bar=foo) etc, then I think your code above will fail - it won't have the right base filters applied, and it might have a different order_by etc.

A test for those cases would be a good addition in any case. There is certainly the possibility for some speedups for the common case.

[anonymous]

Very true. Too bad. However, I still think it'd give a speedup in the very common case that all and get_query_set aren't overridden, won't it? Though I guess this change is less attractive once it gets more tangled with the implementation of Managers.

[Simon Percivall]

Sorry, comment was supposed to be from me.

[Aymeric Augustin]

Accepting based on Luke's comment.

[Anssi Kääriäinen]

It seems #20577 is a duplicate. I think that ticket's patch has better approach.