id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32840	Micro-optimisation possibility in Field.get_col	Keryn Knight	Keryn Knight	"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."	Cleanup/optimization	closed	Database layer (models, ORM)	dev	Normal	fixed			Accepted	1	0	0	0	0	0