#30685: Optimize QuerySet.count() with distinct()
-------------------------------------+-------------------------------------
Reporter: Adam Sołtysik | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: 2.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
The `annotation` part is tracked in #28477 so I'll rename this ticket to
only mention `distinct`.
I'm afraid we can't simply clear the values and ordering, else quite bit
of code in `get_aggregate`
[https://github.com/django/django/blob/65e86948b80262574058a94ccaae3a9b59c3faea/django/db/models/sql/query.py#L440-L444
would be rendundant].
One case that these changes don't consider is the fact that
`.values('foo').distinct()` could influence the `.count()`
e.g.
{{{#!sql
SELECT COUNT(*) FROM (SELECT DISTINCT author_id FROM book)
}}}
Is not equivalent to
{{{#!sql
SELECT COUNT(*) FROM (SELECT DISTINCT book_id FROM book)
}}}
I suggest you open a PR with your changes so CI can report what exactly
that breaks.
I'm getting three failures locally on SQLite; one related to the
aforementioned `.values().distinct()` issue, one related to
`.datetimes().distinct()` and one related to `order_by` spawning a
multiple valued relationship.
--
Ticket URL: <https://code.djangoproject.com/ticket/30685#comment:9>
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