Re: [Django] #17276: Slow anti-join query against Postgres

[Django]

#17276: Slow anti-join query against Postgres
-------------------------------------+-------------------------------------
     Reporter:  dmitry@…             |                    Owner:  nobody
         Type:                       |                   Status:  new
  Cleanup/optimization               |                  Version:  1.3
    Component:  Database layer       |               Resolution:
  (models, ORM)                      |             Triage Stage:  Accepted
     Severity:  Normal               |      Needs documentation:  0
     Keywords:  orm, postgres,       |  Patch needs improvement:  0
  join, anti-join                    |                    UI/UX:  0
    Has patch:  0                    |
  Needs tests:  0                    |
Easy pickings:  0                    |
-------------------------------------+-------------------------------------

Comment (by dberansky):

 I realized over the weekend that the same issue exists for one-to-many
 relationships.  For example, consider these models:

 {{{
 class Parent(models.Model):
     name = models.CharField(max_length=100)

 class Child(models.Model):
     name = models.CharField(max_length=200)
     parents = models.ForeignKey(to=Parent, db_column="parent_id")
 }}}

 and the following Django query:

 {{{
 Parent.objects.filter(child__isnull=True)
 }}}

 for which Django will generate this SQL:


 {{{
 SELECT "demo_parent"."id", "demo_parent"."name"
    FROM "demo_parent"
       LEFT OUTER JOIN "demo_child"
          ON ("demo_parent"."id" = "demo_child"."parent_id")
    WHERE "demo_child"."id" IS NULL

 }}}

 So the same issue as with many-to-many where this query is orders of
 magnitude slower than a slightly modified one:



 {{{
 SELECT "demo_parent"."id", "demo_parent"."name"
    FROM "demo_parent"
       LEFT OUTER JOIN "demo_child"
          ON ("demo_parent"."id" = "demo_child"."parent_id")
    WHERE "demo_child"."parent_id" IS NULL

 }}}

 ('WHERE "demo_child"."__parent_id__" IS NULL' vs. 'WHERE
 "demo_child"."__id__" IS NULL')

-- 
Ticket URL: <https://code.djangoproject.com/ticket/17276#comment:4>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

-- 
You received this message because you are subscribed to the Google Groups 
"Django updates" group.
To post to this group, send email to django-updates@googlegroups.com.
To unsubscribe from this group, send email to 
django-updates+unsubscr...@googlegroups.com.
For more options, visit this group at 
http://groups.google.com/group/django-updates?hl=en.