#18414: queryset.exists() returns False when queryset is distinct and sliced
----------------------------------------------+--------------------
Reporter: bitrut | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.4
Severity: Normal | Keywords: orm
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Consider this:
{{{
User.objects.count()
3
}}}
This queryset
{{{
User.objects.all()[1:].exists()
True
}}}
produces SQL
{{{
SELECT (1) AS "a" FROM "user" LIMIT 1 OFFSET 1
}}}
so the result is correct.
But when you add `distinct()`:
{{{
User.objects.distinct()[1:].exists()
False
}}}
the SQL is
{{{
SELECT DISTINCT (1) AS "a" FROM "user" LIMIT 1 OFFSET 1
}}}
which returns nothing, so the `exists()` returns `False` which is
incorrect.
`DISTINCT` narrows results to just one '''1''' and `OFFSET` omits the
first result and goes to the next one which does not exist.
It's because `DISTINCT` has the higher priority than `OFFSET` in SQL.
I don't know the perfect solution. Maybe in the case of using
`distinct()`, slicing and `exists()` together an exception should be
thrown. Or the `exists()` function should notice this case and call
`count()`, but this would lead to unexpected DB usage.
--
Ticket URL: <https://code.djangoproject.com/ticket/18414>
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