#35947: prefetch_related makes duplicate queries for the same records by
different
relations
-------------------------------------+-------------------------------------
Reporter: Jake Douglas | Owner: (none)
Type: | Status: closed
Cleanup/optimization |
Component: Database layer | Version: 5.1
(models, ORM) |
Severity: Normal | Resolution: wontfix
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Simon Charette):
> For the record, here's an example request from our production
application:
> (313 queries including 279 similar and 242 duplicates)
The example you provided should limit the number of SQL queries to 6 which
you have detailed to be
{{{#!sql
SELECT ••• FROM "houses_house"
SELECT ••• FROM "houses_room" WHERE "houses_room"."house_id" IN (1)
SELECT ••• FROM "houses_window" WHERE "houses_window"."room_id" IN (1) #
DUPLICATE
SELECT ••• FROM "houses_story" WHERE "houses_story"."house_id" IN (1)
SELECT ••• FROM "houses_room" WHERE "houses_room"."story_id" IN (1)
SELECT ••• FROM "houses_window" WHERE "houses_window"."room_id" IN (1) #
DUPLICATE
}}}
Which is one for the top-level houses queryset, two for the
`rooms__windows` prefetch, and three for the `stories__rooms__windows`
prefetch where two are duplicates. As explained above, invasive changes
would be required to take that number to 4 under circumstances that your
specific schema design seems to be exhibiting.
If your application is executing 50x the number of 6 queries that could be
reduced to 4 by implementing this optimization there is likely something
else to look on your side before implementing this optimization. In other
words `prefetch_related` is not a feature that systemically make elevated
number of queries issues go away; it's a tool that bounds the N+1 query
problem and not an identity mapper solution #17.
--
Ticket URL: <https://code.djangoproject.com/ticket/35947#comment:3>
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