On 19/02/12 22:18, Yo-Yo Ma wrote: > It doesn't seem to take advantage ``select_related`` (assuming that > ``topping_type`` is a ``ForeignKey`` from ``Topping``. It instead > sends a separate query for ``Topping`` and ``ToppingType``, joining > them in Python. Is it feasible to modify ``prefetch_related`` use > ``select_related`` when it encounters a ``ForeignKey``? >
It would be possible, but I think a bad idea. There are in fact some valid cases where prefetch_related() is potentially better, even for foreign keys, than for select_related(), for two reasons: 1) the DB could perform worse with the more complex query created by select_related than for two separate queries. 2) the Python-side gains of prefetch_related() could outweigh the advantages of the DB-side gains. To explain (2) a bit more: for foreign keys, prefetch_related() could potentially win quite a bit in terms of memory consumption and CPU time due to the fact that the related objects are shared. If you have table A with 100 rows and a foreign key to table B with 3 rows, and you need the contents of both tables, with select_related you will have to create 100 A objects and 100 B objects, even though most of the B objects are the same - this is just how it works. (This can't be fixed properly without an identity mapper). With prefetch_related, you automatically only get 3 B objects. So, since there are potential performance wins for prefetch_related, this code shouldn't be clever - it should do exactly what the developer asks. This is a good design decision anyway, especially when you are talking about explicit performance related tweaks. If we could automatically tell the right thing to do, neither select_related and prefetch_related would exist at all. Regards, Luke -- "If something is hard, it's not worth doing." (Homer Simpson) Luke Plant || http://lukeplant.me.uk/ -- You received this message because you are subscribed to the Google Groups "Django developers" group. To post to this group, send email to django-developers@googlegroups.com. To unsubscribe from this group, send email to django-developers+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-developers?hl=en.
