Migrations create extra index when foreign key is also part of a unique constraint

[Lev]

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.

[Mariusz Felisiak]

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:

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 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.