#17535: list_detail call to len(queryset) should be queryset.exists()
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Reporter: hedleyroos@… | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Generic views | Version: 1.3
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by claudep):
* needs_better_patch: => 0
* needs_docs: => 0
* needs_tests: => 0
* stage: Unreviewed => Accepted
Comment:
The code you mention is in deprecated code. Moreover, this queryset
evaluation is done in the non paginated section of the context build. The
query, even if large, would be evaluated anyway later in the code. So this
part seems a 'won't fix' for me.
However, the issue is similar in the new class-based view code
(https://code.djangoproject.com/browser/django/trunk/django/views/generic/list.py#L116).
And in this case, the len(queryset)==0 is done before the pagination take
place. So this might be an issue.
If you take the case of a costly query with not so many results, your
proposal would call the query twice (once for exists and once for the real
query), hence resulting in poorer performance when the query is not empty.
Your solution might be better for a quick query that returns many results.
The real solution would be probably to follow the same logic than the old
function-based view and do this test later in the process
(get_context_data).
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Ticket URL: <https://code.djangoproject.com/ticket/17535#comment:1>
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