id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
19029	Reuse QuerySet._result_cache during IN clause	Matt Robenolt	Matt Robenolt	"If I have a `QuerySet` that has been evaluated previously, and shove it into an `__in` clause, a subquery is still generated.

Expected result:[[BR]]
* Reuse a list of primary keys instead, to prevent the database from performing the subquery.

{{{#!python
cities = City.objects.all()
list(cities)  # Force evaluation of cities to populate cache
print cities._result_cache is not None  # Verify that the cache is filled
Event.objects.filter(venue__city__in=cities)  # This should use just the pks instead of a subquery
}}}

I began digging through the ORM, and found the source of the problem being that the origin `QuerySet` object is being clone()'d, causing it's `_result_cache` to be `None`.

The simple solution for now is to not pass a `QuerySet` into an IN clause if we know the set of ids. Can be remedied by something like `[m.pk for m in qs]`, or just `list(qs)`.

For the latter solution, I think the documentation should be updated to reflect this distinction to avoid unexpected results and performance degradation."	Cleanup/optimization	closed	Database layer (models, ORM)	1.4	Normal	wontfix	orm, queryset, result_cache, optimization, subquery	Florian Apolloner charette.s@…	Design decision needed	1	0	0	0	0	0