#24218: Use sub-query in ORM when distinct and order_by columns do not match
-------------------------------------+-------------------------------------
Reporter: miki725 | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: subquery distinct | Triage Stage: Accepted
order_by |
Has patch: 1 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by carljm):
* needs_docs: 0 => 1
* needs_tests: 0 => 1
* stage: Unreviewed => Accepted
Comment:
Thanks for the patch! The feature idea generally makes sense to me -
taking something that currently results in an error and making it instead
return sane results. Tentatively accepting based on that. Some thoughts:
1. We need to check what the other core backends currently do with this
query. If there are backends in which that query is currently an error,
but this patch would change their results, that might be a problem.
2. If I understand the Postgres docs correctly, your proposed subquery SQL
results in an undefined/unpredictable selection of which actual row is
chosen of each group where the `DISTINCT ON` query returns the same value.
I guess if this is a problem, the answer is "make your `ORDER BY` match
the `DISTINCT ON`", but it seems like a non-obvious subtlety that might
bite people.
If nobody else sees any blockers that I'm missing, the things that would
still be needed here are:
a) Add a test demonstrating the new behavior.
b) Add/update the documentation as needed (I think this is a behavior that
ought to be mentioned in the docs somewhere).
c) Turn it into a pull request so the CI runs on it and we can see if it
breaks any existing tests.
Once those are done there may be some comments on code style or
implementation choices. The POC code seems clear enough, but I'd defer to
others who know the ORM better on whether it's actually implemented at the
right level / in the right place.
--
Ticket URL: <https://code.djangoproject.com/ticket/24218#comment:2>
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