#16759: Expensive sql.Query cloning
-------------------------------------+-------------------------------------
Reporter: Suor | Owner: Suor
Type: Bug | Status: new
Component: Database layer | Version: 1.3
(models, ORM) | Resolution:
Severity: Normal | Triage Stage: Accepted
Keywords: orm performance | Needs documentation: 0
cloning | Patch needs improvement: 0
Has patch: 1 | UI/UX: 0
Needs tests: 0 |
Easy pickings: 0 |
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Comment (by akaariai):
An up-to-date patch can be found from
[https://github.com/akaariai/django/compare/ticket_16759].
Some speed measurements using django-bench: nearly all of the query_*
tests are around 20% faster (all except the ones testing large bulk
operation speeds). The qs_filter_chaining is 3x faster.
query_complex_filter is 1.5x faster.
A test case used previously in this ticket:
{{{
10000x
SomeModel.objects.filter(pk=F('pk')+1).all()
}}}
5x faster.
Django's tests suite on sqlite: 10% faster (on an extensive one-iteration
benchmark).
The cloning speed isn't just a testing artefact. Reports for 200ms+
queryset creation times for a single page load have been reported (can't
find these just now, if references are needed, I can dig them from Trac).
The Django ORM uses roughly 4x the time for generating SQL for simple
"SELECT ... WHERE pk = x" query compared to execution of that query. One
large part of this time is queryset cloning speed. In short, the speed
problem is real.
Note that fixing the `__deepcopy__` would make the F() case above faster,
but most of the other speedups are not about the F() case - they are about
the general slower speed of `__deepcopy__` compared to special tailored
.clone() methods.
Add in that the `__deepcopy__` methods Django uses aren't using good
coding practices (they are not doing real deepcopies) and I think we have
pretty convincing evidence that we really do want this approach.
--
Ticket URL: <https://code.djangoproject.com/ticket/16759#comment:33>
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