#30685: Suboptimal QuerySet.distinct().count()
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Reporter: adamsol | Owner: nobody
Type: | Status: closed
Cleanup/optimization |
Component: Database layer | Version: 2.2
(models, ORM) |
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage:
| Unreviewed
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 adamsol):
I don't think it's a bug, but I think there is a place for optimization. I
can't see a reason why all columns are SELECTed in a subquery just to
perform a simple count. I've attached the SQLs generated and what I would
expect.
So, for any model, a query like this:
{{{Model.objects.distinct().count()}}}
should produce an SQL like this:
{{{SELECT COUNT(*) FROM (SELECT DISTINCT "table"."id" AS Col1 FROM
"table") subquery;}}}
instead of:
{{{SELECT COUNT(*) FROM (SELECT DISTINCT "table"."id" AS Col1, ... (all
other columns) FROM "table") subquery;}}}
which, depending on the number of columns and size of the table, can be
several times slower (at least on PostgreSQL).
If you think this is not worth the effort, then OK, I won't insist.
--
Ticket URL: <https://code.djangoproject.com/ticket/30685#comment:2>
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