#19029: Reuse QuerySet._result_cache during IN clause
-------------------------------------+-------------------------------------
Reporter: mattrobenolt | Owner: nobody
Type: | Status: new
Cleanup/optimization | Version: 1.4
Component: Database layer | Keywords: orm, queryset,
(models, ORM) | result_cache, optimization,
Severity: Normal | subquery
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/19029>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To post to this group, send email to django-updates@googlegroups.com.
To unsubscribe from this group, send email to
django-updates+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.
