#35437: Exists subquery ignores ordering when it's distinct
-------------------------------------+-------------------------------------
Reporter: Kevin | Owner: nobody
Marsh |
Type: | Status: new
Uncategorized |
Component: Database | Version: dev
layer (models, ORM) | Keywords:
Severity: Normal | subquery,exists,ordering,distinct
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I ran into a bug in the `Exists` expression, I'm using it to do some
filtering where I just want the latest object grouped by a certain value
(eg. latest financial statement object for each company). See the patch
for a failing test, but using the `Manager`/`Employee` models from the
test suite you can see in the slightly contrived example that for an
`Exists` query like
{{{
# ... filtering by highest paid employee per manager
Exists(
Employee.objects.order_by("manager",
"-salary").distinct("manager").filter(pk=OuterRef("pk"))
)
# ...
}}}
Gets transformed into SQL (Postgresql) like this where the ordering has
been stripped out
{{{
-- ...
EXISTS(
SELECT DISTINCT ON (U0."manager_id")
1 AS "a"
FROM "expressions_employee" U0
WHERE U0."id" = ("expressions_employee"."id")
-- Missing ordering which is required for distinct
LIMIT 1
)
-- ...
}}}
----
Obviously we want to call `clear_ordering` most of the time on `Exists`
subqueries for performance reasons, but in this case we either shouldn't
clear them or loudly raise an exception saying that distinct `Exists`
queries are not supported
--
Ticket URL: <https://code.djangoproject.com/ticket/35437>
Django <https://code.djangoproject.com/>
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