#36606: Optimize QuerySet.values_list(flat=True) with no fields
-------------------------------------+-------------------------------------
Reporter: Adam Johnson | Owner: (none)
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Adam Johnson:
Old description:
> Currently, `QuerySet.values_list()` ensures that no more than 1 field is
> set
> ([https://github.com/django/django/blob/41bc48ac1ed1d515977ebe965993b1ef83eafd02/django/db/models/query.py#L1417-L1421
> source]):
>
> {{{#!python
> if flat and len(fields) > 1:
> raise TypeError(
> "'flat' is not valid when values_list is called with more
> than one "
> "field."
> )
> }}}
>
> However, it also allows the case where *no* fields are declared, for
> which all fields are fetched, only to throw away all but the first one
> ([https://github.com/django/django/blob/41bc48ac1ed1d515977ebe965993b1ef83eafd02/django/db/models/query.py#L266-L278
> source]):
>
> {{{#!python
> class FlatValuesListIterable(BaseIterable):
> """
> Iterable returned by QuerySet.values_list(flat=True) that yields
> single
> values.
> """
>
> def __iter__(self):
> queryset = self.queryset
> compiler = queryset.query.get_compiler(queryset.db)
> for row in compiler.results_iter(
> chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size
> ):
> yield row[0]
> }}}
>
> I think we can optimize this case to select only the first field in the
> model instead, maintaining semantics while avoiding overfetching.
New description:
Currently, `QuerySet.values_list()` ensures that no more than 1 field is
set
([https://github.com/django/django/blob/41bc48ac1ed1d515977ebe965993b1ef83eafd02/django/db/models/query.py#L1417-L1421
source]):
{{{#!python
if flat and len(fields) > 1:
raise TypeError(
"'flat' is not valid when values_list is called with more
than one "
"field."
)
}}}
However, it also allows the case where *no* fields are declared, for which
all fields are fetched, only to throw away all but the first one
([https://github.com/django/django/blob/41bc48ac1ed1d515977ebe965993b1ef83eafd02/django/db/models/query.py#L266-L278
source]):
{{{#!python
class FlatValuesListIterable(BaseIterable):
"""
Iterable returned by QuerySet.values_list(flat=True) that yields
single
values.
"""
def __iter__(self):
queryset = self.queryset
compiler = queryset.query.get_compiler(queryset.db)
for row in compiler.results_iter(
chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size
):
yield row[0]
}}}
I think we can optimize this case to select only the first field in the
model instead, maintaining semantics while avoiding overfetching.
This case also seems untested with the `values_list()` tests in
`tests/lookup/tests.py`, so we'd want to add a test there.
--
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Ticket URL: <https://code.djangoproject.com/ticket/36606#comment:1>
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