#29081: select_related and hitting the database
-------------------------------------+-------------------------------------
Reporter: Дилян | Owner: nobody
Палаузов |
Type: | Status: new
Cleanup/optimization |
Component: | Version: 2.0
Documentation |
Severity: Normal | Keywords: select_related
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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https://docs.djangoproject.com/en/2.0/ref/models/querysets/#select-related
says:
> … then a call to Book.objects.select_related('author_
_hometown').get(id=4) will cache the related Person and the related City:
{{{
b = Book.objects.select_related('author_ _hometown').get(id=4)
p = b.author # Doesn't hit the database.
c = p.hometown # Doesn't hit the database.
b = Book.objects.get(id=4) # No select_related() in this example.
p = b.author # Hits the database.
c = p.hometown # Hits the database.
}}}
This leaves the impression, that the first snipped does not hit the
database and the second snippet hits twice the database.
I propose:
* wring after {{{b = Book.objects.select_related('author_
_hometown').get(id=4)}}} the comment # Hits the database with INNER JOIN.
* changing {{{b = Book.objects.get(id=4) # No select_related() in this
example.}}} to {{{b = Book.objects.get(id=4) # No select_related() in this
example, hits the database.}}}, and
* changing {{{p = b.author # Hits the database.}}} to {{{p =
b.author # Doesn't hit the database.}}}
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Ticket URL: <https://code.djangoproject.com/ticket/29081>
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