#18702: Remove chunked reads from iter(qs)
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Reporter: akaariai | Owner: nobody
Type: | Status: new
Cleanup/optimization | Version: 1.4
Component: Database layer | Resolution:
(models, ORM) | Triage Stage: Design
Severity: Normal | decision needed
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by akaariai):
A rebased + squashed patch available from:
https://github.com/akaariai/django/commit/ddc85150d0bb82231e3820807f7652d2a9578ab2
There were some changes to the queryset iteration lately (the introduction
of _safe_iterator()). So, currently there are six nested iterators when
iterating over a qs. This is getting a bit complex.
Performance difference between the patched version and master for fetching
ten objects from the DB using "for obj in qs" is 5%-7%. For
values_list('pk') it is around 15%. So, there is a performance difference
but it isn't too big.
The silent discard of some exceptions should be fixed with the
_safe_iterator() patch.
Still, the code clarity difference is quite big. When considering this
patch the question is mainly if there are some use cases where the
"convert only some results to Python" is a big performance win. I don't
believe such cases are common enough to care about, but then again I know
mostly my own usage patterns... To me it seems that in most cases where
the change would matter the user is doing something that will cause bad
performance anyways.
--
Ticket URL: <https://code.djangoproject.com/ticket/18702#comment:2>
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