Postgres hash exclusion constraints are not considered totally unique

[Tilman Koschnick]

Hi,

with Postgres, a hash index can be used as an exclusion constraint, functioning the same way a unique btree based constrained. Support for this has recently been merged to Django: https://code.djangoproject.com/ticket/36827

Using this on a USERNAME_FIELD can be good for index size and performance. Currently, the auth system checks doesn't detect a hash exclusion constraint as a viable option though:

account.User: (auth.E003) 'User.email' must be unique because it is named as the 'USERNAME_FIELD'.

The attached patch enhances the check to consider hash constraints as well.

Cheers, Til

[Simon Charette]

Accepting on the basis that the check logic should be able to consider all type of unique constraints but the proposed patch is inadequate.

What should be done instead is find a way to have django.db.models.options.Options.total_unique_constraints include such constraint in a way that doesn't couple contrib.postgres with core.

A potential solution could be to introduce a Constraint.totally_unique_for_fields: set[tuple[str]] | None property which is implemented by UniqueConstraint and ExclusionConstraint ​over the introspection logic that currently exists.

That might call for a Options.unique_together_fields: set[tuple[str]] that combine Field.unique, Options.unique_together and Constraint.totally_unique_for_fields so interested parties don't have to check of them every time.