#23319: Django uses unnecessary join in concrete inheritance
----------------------------------------------+--------------------
Reporter: avidi | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer (models, ORM) | Version:
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
----------------------------------------------+--------------------
In some situations django use JOIN to get data from parent model when it's
unnecessary.
== First case ==
Using select to get only fields of children.
{{{
class Parent(models.Model):
field_of_parent = models.TextField()
class Child(Parent):
field_of_child = models.TextField()
}}}
{{{
>>> str(Child.objects.only("field_of_child").all().query)
'SELECT "homettt_child"."parent_ptr_id", "homettt_child"."field_of_child"
FROM "homettt_child" INNER JOIN "homettt_parent" ON (
"homettt_child"."parent_ptr_id" = "homettt_parent"."id" )'
}}}
== Second case ==
Get all fields but parent hasn't any field
{{{
class EmptyParent(models.Model):
pass
class ChildB(EmptyParent):
field_of_child = models.TextField()
}}}
{{{
>>> str(ChildB.objects.all().query)
'SELECT "homettt_emptyparent"."id", "homettt_childb"."emptyparent_ptr_id",
"homettt_childb"."field_of_child" FROM "homettt_childb" INNER JOIN
"homettt_emptyparent" ON ( "homettt_childb"."emptyparent_ptr_id" =
"homettt_emptyparent"."id" )'
}}}
== Third case ==
In similar way through relations:
For example, ManyToMany relations django could use _ptr_id column to join
but uses id column of parent. If you don't need any fields (or they don't
exist) the join is unnecessary.
{{{
class Parent(models.Model):
field_of_parent = models.TextField()
class Child(Parent):
field_of_child = models.TextField()
class OtherModel(models.Model):
m2mrelation = models.ManyToManyField(Child)
}}}
{{{
other=OtherModel.objects.create()
>>> other.m2mrelation.create()
<Child: Child object>
str(other.m2mrelation.only("field_of_child").all().query)
>>> str(other.m2mrelation.only("field_of_child").all().query)
'SELECT "homettt_child"."parent_ptr_id", "homettt_child"."field_of_child"
FROM "homettt_child" INNER JOIN "homettt_othermodel_m2mrelation" ON (
"homettt_child"."parent_ptr_id" =
"homettt_othermodel_m2mrelation"."child_id" ) INNER JOIN "homettt_parent"
ON ( "homettt_child"."parent_ptr_id" = "homettt_parent"."id" ) WHERE
"homettt_othermodel_m2mrelation"."othermodel_id" = 1'
}}}
Maybe these could be solved using _ptr_id as "default pk" in child models
and only use id of parent if is necessary information of table's parent
model.
Tested in 1.7rc2 but maybe this appears in previous versions.
--
Ticket URL: <https://code.djangoproject.com/ticket/23319>
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 unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/048.8c8bbe0b320f45a4796d5e479799dbd2%40djangoproject.com.
For more options, visit https://groups.google.com/d/optout.
