#10790: Too many joins in a comparison for NULL.
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Reporter: mtredinnick | Owner: mtredinnick
Status: new | Milestone:
Component: ORM aggregation | Version: SVN
Keywords: | Stage: Unreviewed
Has_patch: 0 |
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Using these models:
{{{
#!python
class Student(models.Model):
name = models.CharField(max_length=50)
class Message(models.Model):
title = models.CharField(max_length=50)
student = models.ForeignKey(Student, null=True)
}}}
this query generates slightly inefficient SQL:
{{{
#!python
Message.objects.filter(student=None)
}}}
We get this
{{{
#!sql
SELECT `outer_message`.`id`, `outer_message`.`title`,
`outer_message`.`student_id`
FROM `outer_message`
LEFT OUTER JOIN `outer_student`
ON (`outer_message`.`student_id` = `outer_student`.`id`)
WHERE `outer_student`.`id` IS NULL
}}}
when we should ideally be using this:
{{{
#!sql
SELECT `outer_message`.`id`, `outer_message`.`title`,
`outer_message`.`student_id`
FROM `outer_message`
WHERE `outer_message`.`student_id` IS NULL
}}}
Not worth worrying about for 1.1, since the result isn't incorrect; just
less efficient than it could be.
Note to self for when fixing this: This is essentially the only situation
where we can trim a left outer join from the end of a list of joins. We're
comparing to NULL and can factor that back across the outer join. Needs
special casing in `Query.add_filter()`.
--
Ticket URL: <http://code.djangoproject.com/ticket/10790>
Django <http://code.djangoproject.com/>
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