#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
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Comment (by carljm):
Replying to [comment:3 ramiro]:
> Per the Postgres documentation snipped, shouldn't the `ORDER BY`
actually be applied to the sub-query to ensure a predictable result?
I think the idea is that in this case the user has supplied an `ORDER BY`
which _can't_ be applied to the sub-query (because it doesn't match the
`DISTINCT ON`), and so we accept unpredictable results in order to provide
some results which match what the user requested.
This business of unpredictable results is what makes me most uncomfortable
with the patch.
Would it be better to auto-generate an `ORDER BY` for the subquery
matching the `DISTINCT ON`?
If we did that, the subquery isn't even needed: we could get the same
results by just automatically prepending the `ORDER BY` such that it
matches the `DISTINCT ON`.
Not sure if that's a good idea, but it seems like perhaps at least as good
an idea as the one proposed here.
--
Ticket URL: <https://code.djangoproject.com/ticket/24218#comment:4>
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