Hi,
I don't use extra() a lot, but it could be that count() clears out extra.
This doesn't exactly answer your question, but you may want to try your
query on the 1.8 alpha using the new available expressions. You might be
able to do this without needing extra() at all in 1.8.
Collin
On Sunday, January 18, 2015 at 3:24:49 PM UTC-5, Mattias Linnap wrote:
>
> Hi all,
>
> I think I've found a strange case where QuerySet.count() does not match
> len(queryset), and the behaviour is certainly unexpected.
> But I'm not sure whether this is a bug, some mistake on my part, or a
> known limitation of combining .extra(), .distinct() and .count().
> I am aware of the default ordering interfering with .distinct(), and
> already add .order_by() to get rid of it.
>
> I have a model called RadioSignal, with an integer field "rssi". I'm
> interested in finding out how many distinct values for "rssi / 10" there
> are (-10, -20, -30, etc).
>
> Here is a commented "./manage.py shell" record:
>
> >>> RadioSignal.objects.count()
> 523
> >>> RadioSignal.objects.order_by().values('rssi').distinct().count()
> 49
> >>> connection.queries[-1]['sql']
> 'SELECT COUNT(DISTINCT "maps_radiosignal"."rssi") FROM "maps_radiosignal"'
>
> Looks okay so far. But I'm interested in each distinct tens of RSSI
> values, not every single value. I can compute these with .extra():
>
> >>> len(RadioSignal.objects.order_by().extra({'tens': 'rssi /
> 10'}).values('tens').distinct())
> 6
> >>> RadioSignal.objects.order_by().extra({'tens': 'rssi /
> 10'}).values('tens').distinct()
> [{'tens': -8}, {'tens': -4}, {'tens': -5}, {'tens': -9}, {'tens': -6},
> {'tens': -7}]
> >>> connection.queries[-1]['sql']
> 'SELECT DISTINCT (rssi / 10) AS "tens" FROM "maps_radiosignal" LIMIT 21'
>
> Also looks good so far. But running len() on a queryset is unnecessary if
> I only need the count.
>
> >>> RadioSignal.objects.order_by().extra({'tens': 'rssi /
> 10'}).values('tens').distinct().count()
> 523
> >>> connection.queries[-1]['sql']
> 'SELECT COUNT(DISTINCT "maps_radiosignal"."id") FROM "maps_radiosignal"'
>
> Uhoh. Somehow .count() keeps the .distinct() part, but replaces the
> .extra() and .values() parts with counting primary keys?
>
> I tried it with values('tens'), values_list('tens'), and
> values_list('tens', flat=True), as well no change.
> So far I've tested Django 1.6 with Python 2.7 and PostgreSQL 9.3, and
> Django 1.7 with Python 3.4 and PostgreSQL 9.1, all seem to behave the same.
>
> I can work around this, since the possible resulting querysets are pretty
> small, and evaluating them with len() isn't too slow. But I'm wondering if
> it's a bug in Django, or something else I've missed?
>
> Mattias
>
>
>
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