#35102: Performance regression on Model.objects.count() between Django 4.2 and
5.0
-------------------------------------+-------------------------------------
Reporter: Anthony Shaw | Owner: (none)
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: 5.0
(models, ORM) |
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
-------------------------------------+-------------------------------------
Comment (by Simon Charette):
> I suspect this would impact other benchmarks as well, hash functions
should be as fast as possible but I understand why that is challenging for
something like expressions. inspect.signature is a very slow function that
does a huge amount of processing to build a Signature object for the given
callable.
yeah that's the crux of the issue here. This would require having each
expression implement their own correct identity functions based on their
`__init__` signature which would be a significant maintenance burden and
also backward incompatible for third party expressions.
I think we can get a lot of benefit by ''caching'' `inspect.signature`
calls like the following.
{{{#!diff
diff --git a/django/db/models/expressions.py
b/django/db/models/expressions.py
index c20de5995a..0703a134a9 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -485,15 +485,22 @@ def select_format(self, compiler, sql, params):
class Expression(BaseExpression, Combinable):
"""An expression that can be combined with other expressions."""
+ def __init_subclass__(cls, /, **kwargs):
+ super().__init_subclass__(**kwargs)
+ cls._constructor_signature = inspect.signature(cls.__init__)
+
+ _constructor_signature = inspect.signature(BaseExpression.__init__)
+
@cached_property
def identity(self):
- constructor_signature = inspect.signature(self.__init__)
args, kwargs = self._constructor_args
- signature = constructor_signature.bind_partial(*args, **kwargs)
+ signature = self._constructor_signature.bind_partial(self, *args,
**kwargs)
signature.apply_defaults()
arguments = signature.arguments.items()
identity = [self.__class__]
for arg, value in arguments:
+ if arg == 'self':
+ continue
if isinstance(value, fields.Field):
if value.name and value.model:
value = (value.model._meta.label, value.name)
}}}
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Ticket URL: <https://code.djangoproject.com/ticket/35102#comment:10>
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