Hi,
Here are some performance numbers against a local SQLite in case any
one is interested. GL has about 40,000 records for reference.324 was
just a random number chosen, of different 'in's
>>> sys.version'3.7.2rc1 (default, Dec 12 2018, 06:25:49) \n[GCC
8.2.0]'>>> django.__version__'3.0.6'>>> import myapp.models as m>>> x =
m.GL.objects.all()[324]>>> x in m.GL.objects.all()True
>>> import timeit>>> timeit.timeit('m.GL.objects.filter(pk=x.pk)',
setup='import myapp.models as m;x = m.GL.objects.all()[324]',
number=100)0.05818330496549606>>> timeit.timeit('x in qs',
setup='import myapp.models as m;qs = m.GL.objects.all(); x=qs[324]',
number=100)1.5688817161135375
On Tue, 2020-06-02 at 05:53 -0700, Tim Graham wrote:
> It may help to know that QuerySet.__contains__() was implemented
> until Django 1.6 when chunked reads were removed from QuerySet
> iteration:
>
> https://github.com/django/django/commit/70679243d1786e03557c28929f9762a119e3ac14
> On Tuesday, June 2, 2020 at 7:43:25 AM UTC-4, Aymeric Augustin wrote:
> > > On 2 Jun 2020, at 13:30, Florian Apolloner <f.apo...@gmail.com>
> > > wrote:
> > >
> > > On Tuesday, June 2, 2020 at 11:28:34 AM UTC+2, Adam Johnson
> > > wrote:
> > > > > If you already fetched the queryset, `if obj in queryset`
> > > > > will make a new database query
> > > >
> > > > I don't see why we couldn't implement __contains__ to do a walk
> > > > through _result_cache if it has been fetched?
> > > >
> > >
> > > we are doing the same for len() IIRC. I have to say it sometimes
> > > makes it a little bit hard to debug, but if it is documented as
> > > such it should be okay.
> >
> > These are good points.
> >
> > As far as I can tell, the current implementation of __len__ is to
> > fetch the whole queryset:
> >
> > def __len__(self):
> > self._fetch_all()
> > return len(self._result_cache)
> >
> > I'm almost sure doing an automatic .count() was discussed in the
> > past. I don't remember why we rejected it — perhaps because in most
> > cases, when you need the length of a queryset, you will also
> > iterate it?
> >
> > The suggestion for __contains__ is similar: doing an automatic
> > .filter(pk=obj.pk).exists(). We could do it only if the queryset
> > isn't fetched yet, but I'm afraid this may be too magical. Is it
> > reasonable to have `total = len(queryset); found = obj in queryset`
> > make one SQL query while `found = obj in queryset; total =
> > len(queryset)` makes two queries?
> >
> > Now I'm worried that we may be making inconsistent decisions
> > between __len__ and __contains__... Let's make sure we're
> > consistent across dunder methods.
> >
> > --
> > Aymeric
> >
> >
>
>
>
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