#26434: Inconsistent results of QuerySet count() method using PostgreSQL backend
prior and post the QuerySet evaluation
-------------------------------------+-------------------------------------
Reporter: kamandol | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.9
(models, ORM) | Keywords: postgresql queryset
Severity: Normal | count annotate aggreagate order_by
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Using a PostgreSQL backend database, in some cases with QuerySets
involving '''annotations''', '''values''' or '''values_list''' and
ordering('''order_by''') clauses the resulting QuerySet '''count()'''
method fails to predict the real row result unless that QuerySet is
evaluated.
For instance:
{{{
#!python
q = BundleVersion.objects.order_by('-id').values_list("port__id",
"asset_bundle__id").annotate(max=models.Max("id"))
}}}
This QuerySet using PostgreSQL would, in fact, group the results by
'''BundleVersion.id''' instead of the tuple ("port!__id",
"asset_bundle!__id") due to the '''order_by''' clause using a column not
in the '''values_list'''. This is a documented behavior.
The problem is that if '''q''' is not evaluated yet, calling
'''q.count()''' will return an amount of rows as if the grouping was done
on the tuple ("port!__id", "asset_bundle!__id"). '''After''' evaluating
'''q''', or calling '''len(q)''', the result of '''q.count()''' will be
the expected, as if the grouping was done using ''''BundleVersion.id''''.
--
Ticket URL: <https://code.djangoproject.com/ticket/26434>
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