#35309: Elide ordering of prefetch querysets for single valued relationships
-------------------------------------+-------------------------------------
Reporter: Laurent Lyaudet | Owner: Laurent
Type: | Lyaudet
Cleanup/optimization | Status: assigned
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Normal | Resolution:
Keywords: prefetch single- | Triage Stage: Accepted
valued order_by |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Laurent Lyaudet:
Old description:
> While the ordering of multi-valued relationships must be preserved when
> prefetching relationships is it unnecessary when using `prefetch_related`
> against single valued relationships.
>
> For example, given the following models
>
> {{{#!python
> class Author(models.Model):
> name = models.CharField(max_length=200)
>
> class Meta:
> ordering = ["name"]
>
> class Book(models.Model):
> title = models.CharField(max_length=200)
> author = models.ForeignKey(Author, related_name="books",
> on_delete=models.CASCADE)
>
> class Meta:
> ordering = ["title"]
> }}}
>
> The ordering of an author's books in
> `Author.objects.prefetch_related("books")` has a significance as multiple
> books might be associated with each authors.
>
> It's not the case for a book's author in
> `Book.objects.prefetch_related("author")` through as the relationship can
> only contain a single author and there is a single way to order the
> members of a singleton.
>
> In other words `sorted([element], key=sort_func)` will result in
> `[element]` for any `sort_func`.
>
> This property holds true for all the single valued relationships that the
> ORM supports (backward and forward 1:1 and forward 1:M) which allows the
> prefetching to elide any predefined ordering safely to avoid an
> unnecessary and possibly expensive ordering defined for the related model
> queryset.
> Currently the prefetch of authors will use the order by and add useless
> charge on the DB server.
> It would be useful to remove this order by.
> #ClimateChangeBrake
New description:
While the ordering of multi-valued relationships must be preserved when
prefetching relationships is it unnecessary when using `prefetch_related`
against single valued relationships.
For example, given the following models
{{{#!python
class Author(models.Model):
name = models.CharField(max_length=200)
class Meta:
ordering = ["name"]
class Book(models.Model):
title = models.CharField(max_length=200)
author = models.ForeignKey(Author, related_name="books",
on_delete=models.CASCADE)
class Meta:
ordering = ["title"]
}}}
The ordering of an author's books in
`Author.objects.prefetch_related("books")` has a significance as multiple
books might be associated with each authors.
It's not the case for a book's author in
`Book.objects.prefetch_related("author")` through as the relationship can
only contain a single author and there is a single way to order the
members of a singleton.
In other words `sorted([element], key=sort_func)` will result in
`[element]` for any `sort_func`.
This property holds true for all the single valued relationships that the
ORM supports (backward and forward 1:1 and forward 1:M) which allows the
prefetching to elide any predefined ordering safely to avoid an
unnecessary and possibly expensive ordering defined for the related model
queryset.
Currently the prefetch of authors will use the order by and add useless
load on the DB server.
It would be useful to remove this order by.
#ClimateChangeBrake
--
--
Ticket URL: <https://code.djangoproject.com/ticket/35309#comment:13>
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