Opened 3 years ago
Closed 3 years ago
#32838 closed Cleanup/optimization (wontfix)
Migrations create extra index when foreign key is also part of a unique constraint
Reported by: | Lev | Owned by: | nobody |
---|---|---|---|
Component: | Database layer (models, ORM) | Version: | 3.2 |
Severity: | Normal | Keywords: | postgres, indexes, unique constraint, foreign keys |
Cc: | Triage Stage: | Unreviewed | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
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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Thanks for this ticket, however it is a documented behavior. It's also documented how to disable
ForeignKey
indexes if you want to avoid the overhead:There are also valid cases for a separate index (see 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.