#15819: Admin searches should use distinct, if query involves joins
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Reporter: Adam Kochanowski | Owner: ryankask
<aip@…> | Status: reopened
Type: Bug | Version: master
Component: contrib.admin | Resolution:
Severity: Normal | Triage Stage: Accepted
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by hcarvalhoalves):
The problem, though, is that `DISTINCT` is '''painfully slow''' on most
databases. A query like...
{{{
SELECT DISTINCT "mytable"."id", ... ORDER BY "mytable"."somefield" DESC
LIMIT 100
}}}
..will force a full table scan and query time will increase linearly with
the number of rows. Django shouldn't ''guess'' when to use it internally
without a way for client code to override it, ''specially'' if this logic
is moved to the ORM.
This currently happens on `contrib.admin` if you include a M2M relation in
the `list_filters`, and makes the `changelist` views unusable if your
database has more than a couple hundred queries. See ticket #18729 for
that.
More often than not, you want to optimize the query to use a subquery
(giving hints to the query planner because it increases selectivity), or
querying ''without'' `DISTINCT` and removing duplicates on Python side.
Here's a case study on optimizing queries to avoid use of `DISTINCT`:
http://www.databasejournal.com/features/postgresql/article.php/3437821
/SELECT-DISTINCT-A-SQL-Case-Study.htm
--
Ticket URL: <https://code.djangoproject.com/ticket/15819#comment:19>
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