#26925: ORM changes grouping when default ordering is set
----------------------------------------------+-------------------------
Reporter: njoyard | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.8
Severity: Normal | Keywords: aggregation
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Consider the following model definition:
{{{
class Thing(Model):
foo = SomeField()
bar = SomeField()
class Meta:
ordering = ('foo',)
}}}
Imagine I would like to count instances of Things for each 'bar' value:
{{{
qs = Thing.objects.all.values('bar').annotate(cnt=Count('bar'))
}}}
I would expect the ORM to execute something along the lines of
{{{
SELECT bar, COUNT(bar) FROM things GROUP BY bar
}}}
Except the ORM wants to enforce the default ordering, and the only way to
do so is to execute
{{{
SELECT bar, COUNT(bar) FROM things GROUP BY foo, bar ORDER BY foo
}}}
...which is a completely different query. As a user of the model, I have
to add an ordering override (that I didn't need and that adds an ordering
step in the query execution plan):
{{{
qs =
Thing.objects.all.values('bar').annotate(cnt=Count('bar')).order_by('bar')
}}}
I would expect the ORM to either:
- ignore the default ordering, given that it uses fields that are no
longer retrieved in the query
- fail executing the query with an exception stating that my aggregation
is not compatible with the model default ordering
--
Ticket URL: <https://code.djangoproject.com/ticket/26925>
Django <https://code.djangoproject.com/>
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