[Django] #32840: Micro-optimisation possibility in Field.get_col
Django
-------------------------------------+-------------------------------------
Reporter: Keryn | Owner: nobody
Knight |
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: dev
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Current implementation is:
{{{
def get_col(self, alias, output_field=None):
if output_field is None:
output_field = self
if alias != self.model._meta.db_table or output_field != self:
from django.db.models.expressions import Col
return Col(alias, self, output_field)
else:
return self.cached_col
}}}
If no ''different'' output field is provided, is doing the following
comparison needlessly as far as I can tell: `output_field != self` for
which
the default implementation of `Field.__eq__` is:
{{{
if isinstance(other, Field):
return (
self.creation_counter == other.creation_counter and
getattr(self, 'model', None) == getattr(other, 'model',
None)
)
return NotImplemented
}}}
in that scenario, because `self` and `output_field` are literally the same
object (down to the `id(...)`) the `isinstance` resolves to True, the
creation counters also are the same and the models are as you'd expect ...
the same. There's no short-circuiting via falsy condition available.
I think that the method body can be changed to:
{{{
has_diff_output_field = True
if output_field is None:
output_field = self
has_diff_output_field = False
if alias != self.model._meta.db_table or (has_diff_output_field and
output_field != self):
from django.db.models.expressions import Col
return Col(alias, self, output_field)
else:
return self.cached_col
}}}
The introduction of `has_diff_output_field` being the important part. If
it's `False` then comparison short-circuiting will prevent the execution
of `output_field != self` at all.
I'm purposefully avoiding making further investigation/judgement about
whether `output_field != self` is itself necessary, because it's
ostensibly possible for a custom `output_field` to be provided which has
the same `creation_counter` + `model` and I don't know how ''likely'' that
is.
Across the entire test suite (ignoring those which have skipped), executed
with the proposed change didn't seem to break anything (yay) and
augmenting the method additionally with:
{{{
if has_diff_output_field:
print('different')
else:
print('same')
}}}
and counting the results across some 14K tests, there were `87021
different` and `178493 same`.
Quick example of how to get to the method:
{{{
>>> tuple(get_user_model().objects.all())
(Pdb) w
/path/django/db/models/query.py(280)__iter__()
-> self._fetch_all()
/path/django/db/models/query.py(1343)_fetch_all()
-> self._result_cache = list(self._iterable_class(self))
/path/django/db/models/query.py(51)__iter__()
-> results = compiler.execute_sql(chunked_fetch=self.chunked_fetch,
chunk_size=self.chunk_size)
/path/django/db/models/sql/compiler.py(1175)execute_sql()
-> sql, params = self.as_sql()
/path/django/db/models/sql/compiler.py(523)as_sql()
-> extra_select, order_by, group_by = self.pre_sql_setup()
/path/django/db/models/sql/compiler.py(55)pre_sql_setup()
-> self.setup_query()
/path/django/db/models/sql/compiler.py(46)setup_query()
-> self.select, self.klass_info, self.annotation_col_map =
self.get_select()
/path/django/db/models/sql/compiler.py(228)get_select()
-> cols = self.get_default_columns()
/path/django/db/models/sql/compiler.py(715)get_default_columns()
-> column = field.get_col(alias)
> /path/django/db/models/fields/__init__.py(396)get_col()
}}}
Overall it's:
- 1 comparison if they're the same (it was 1 comparison before too, but
that was itself 3 comparisons)
- 1 additional comparison if they're not the same.
The weighting/ratio of the test suite + the fact that the ''simplest'' ORM
usage suggests (to me) it might have merit.
Addendum: when I say micro, I mean micro. It's not a big time saver, I
just happened to notice upon far more calls to `__eq__` than I expected.
--
Ticket URL: <https://code.djangoproject.com/ticket/32840>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
Django
-------------------------------------+-------------------------------------
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: dev
(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
-------------------------------------+-------------------------------------
* stage: Unreviewed => Accepted
Comment:
Hi Keryn, thanks. I'm going to provisionally Accept this to let
Mariusz/Simon have a look at it in PR.
I wonder if we need the initial conditional at all... 🤔
Untested, but same idea I think:
{{{
def get_col(self, alias, output_field=None):
if alias != self.model._meta.db_table or (output_field and
output_field != self):
from django.db.models.expressions import Col
return Col(alias, self, output_field or self)
else:
return self.cached_col
}}}
> I'm purposefully avoiding making further investigation/judgement about
whether output_field != self is itself necessary, because it's ostensibly
possible for a custom output_field to be provided which has the same
creation_counter + model and I don't know how likely that is.
Yes.
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:1>
Django
-------------------------------------+-------------------------------------
Type: | Knight
Cleanup/optimization | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* owner: nobody => Keryn Knight
* status: new => assigned
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:2>
Django
-------------------------------------+-------------------------------------
Type: | Knight
Cleanup/optimization | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* has_patch: 0 => 1
Comment:
PR is here: https://github.com/django/django/pull/14585
For consistency across 2 separate comment areas:
The reason I've not opted for the suggested alternative above (and have
instead kept my rough original proposal) is not a criticism of it, but for
keeping intent as clear as possible -- the version which was suggested
relies on the mental parsing of the `x or y` twice (which always causes
''me'' to double take) and leaves open an accidental regression in the
future should someone justify an implementation of `Field.__bool__` which
is equally/more costly as `Field.__eq__` (I think).
(Obviously I can change that stance, if it's even deemed worthwhile doing
it. We'll see)
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:3>
Django
-------------------------------------+-------------------------------------
Type: | Knight
Cleanup/optimization | Status: assigned
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* needs_better_patch: 0 => 1
Comment:
Updating state based on initial review on the PR.
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:4>
Django
-------------------------------------+-------------------------------------
Type: | Knight
Component: Database layer | Version: dev
(models, ORM) |
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* status: assigned => closed
* resolution: => fixed
Comment:
In [changeset:"5013798fe9a87b693ddb5589a8a408a04a873781" 5013798f]:
{{{
#!CommitTicketReference repository=""
revision="5013798fe9a87b693ddb5589a8a408a04a873781"
Fixed #32840 -- Optimized Field.get_col().
get_col() used "self" as "output_field" when it was not given, and
unnecessarily compared "self" to "self".
Co-authored-by: Chris Jerdonek <chris.j...@gmail.com>
Co-authored-by: Mariusz Felisiak <felisiak...@gmail.com>
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:5>
Django
-------------------------------------+-------------------------------------
Type: | Knight
Cleanup/optimization | Status: closed
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution: fixed
Keywords: | Triage Stage: Accepted
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* needs_better_patch: 1 => 0
--
Ticket URL: <https://code.djangoproject.com/ticket/32840#comment:6>