#16773: QuerySet.count does no caching until the result cache is filled
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Reporter: | Owner: nobody
eternicode | Status: new
Type: Bug | Component: Database layer
Milestone: | (models, ORM)
Version: 1.3 | Severity: Normal
Resolution: | Keywords:
Triage Stage: | Has patch: 0
Unreviewed | Needs tests: 0
Needs documentation: 0 | Easy pickings: 0
Patch needs improvement: 0 |
UI/UX: 0 |
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Changes (by tarequeh):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
Currently call to count stops hitting DB once the queryset is cached. If a
cache for count is introduced, count will stop hitting DB upon on a call
to count. And that may happen before queryset get evaluated/cached.
Afterwards when queryset is evaluated, it might have number of results
different than the cached count depending on how many objects were created
between the call to count and evaluation. So codes like this won't be
valid anymore:
{{{
#!div style="font-size: 80%"
{{{#!python
books = Book.objects.all()
print books.count() # say prints 5
Book.objects.create(title='Game of Thrones')
print books.count() # currently prints 6 but with cache will print 5
}}}
}}}
But if the queryset is fully cached, then calling .count doesn't invoke
further count calls:
{{{
#!div style="font-size: 80%"
{{{#!python
books = Book.objects.all()
print books.count() # say prints 5
print [x.__dict__ for x in books]
Book.objects.create(title='Game of Thrones')
print books.count() # prints 5
}}}
}}}
So caching count is probably not a good idea.
--
Ticket URL: <https://code.djangoproject.com/ticket/16773#comment:1>
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