Re: [Django] #32838: Migrations create extra index when foreign key is also part of a unique constraint

[Django]

#32838: Migrations create extra index when foreign key is also part of a unique
constraint
-+-
 Reporter:  Lev  |Owner:  nobody
 Type:   |   Status:  closed
  Cleanup/optimization   |
Component:  Database layer   |  Version:  3.2
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  wontfix
 Keywords:   | Triage Stage:
  postgres,indexes,unique|  Unreviewed
  constraint,foreign keys|
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Mariusz Felisiak):

 * status:  new => closed
 * resolution:   => wontfix


Comment:

 Thanks for this ticket, however it is a
 [https://docs.djangoproject.com/en/3.2/ref/models/fields/#foreignkey
 documented] behavior. It's also documented how to disable `ForeignKey`
 indexes if you want to avoid the overhead:

  ''A database index is automatically created on the `ForeignKey`. You can
 disable this by setting `db_index` to `False`. You may want to avoid the
 overhead of an index if you are creating a foreign key for consistency
 rather than joins, or if you will be creating an alternative index like a
 partial or **multiple column index**.''

 There are also valid cases for a separate index (see
 [https://code.djangoproject.com/ticket/22125#comment:17 comment]). IMO we
 shouldn't break this.

 You can raise the idea on the DevelopersMailingList to reach a wider
 audience and see what other think. We can re-open this ticket if we reach
 consensus on the mailing list.

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[Django] #32838: Migrations create extra index when foreign key is also part of a unique constraint

[Django]

#32838: Migrations create extra index when foreign key is also part of a unique
constraint
-+-
   Reporter:  levkk  |  Owner:  nobody
   Type: | Status:  new
  Cleanup/optimization   |
  Component:  Database   |Version:  3.2
  layer (models, ORM)|   Keywords:
   Severity:  Normal |  postgres,indexes,unique
   Triage Stage: |  constraint,foreign keys
  Unreviewed |  Has patch:  0
Needs documentation:  0  |Needs tests:  0
Patch needs improvement:  0  |  Easy pickings:  0
  UI/UX:  0  |
-+-
 Given a model like so:


 {{{
 class Person(models.Model):
 name = models.TextField()

 class Address(models.Model):
 person = models.ForeignKey(Person, on_delete=models.PROTECT)
 street = models.TextField()

 class Meta:
unique_together = ["person", "street"]
 }}}

 and a Postgres backend, Django will create two btree indexes, one on the
 foreign key "person" and another on "person, street" to enforce the unique
 constraint. In this case, the "person" index is redundant and wasteful
 since the "person, street" index is sufficient. We can check for the
 foreign key index being covered by the unique index and not create it if
 that's the case.

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