#26780: Prefetch objects don't work with slices
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Reporter: ludwik | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 1.9
Severity: Normal | Resolution:
Keywords: prefetch, slicing | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by ludwik):
Charettes,
Thank you for the reply.
> accessing category.example_posts[:3] would have the same effect
I'm pretty sure that is not correct. The end effect would obviously be the
same, but the way it is achieved behind the scenes and the performance
characteristics that follows would be widely different. What you propose
(prefetching without limit, adding slicing in a loop after the fact) would
make Django to perform a database query selecting '''all objects from the
database table''' and loading them into memory. This would happen when the
main queryset is evaluated (that's what prefetching is all about). Then
the slicing would be perform by Python in memory, on a queryset that was
already evaluated.
That's what I understood from
[https://docs.djangoproject.com/en/1.9/ref/models/querysets/#prefetch-
objects the documentation] and also how Django actually behaved in an
experiment I performed couple of minutes ago. What I want to avoid is
exactly this behavior - loading thousands of objects from the database to
display first three of them.
I would be happy with a sane workaround, but to my knowledge there isn't
any.
--
Ticket URL: <https://code.djangoproject.com/ticket/26780#comment:4>
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