Unique indexes

[David Sanders]

Django already supports both unique constraints and indexes from the models Meta <3

A worthwhile addition would be to support _unique indexes_, if the database makes this distinction.

In Postgres a unique index is a distinct feature from unique constraints; unique indexes are the mechanism that enforces the unique constraints declared for the table: ​https://www.postgresql.org/docs/current/indexes-unique.html.

One important difference between the 2 is the fact that indexes may support expressions whereas constraints do not (at least in PG).

An example may be to enforce a unique attribute in a jsonb field:

CREATE UNIQUE INDEX unique_attr ON some_table ( (some_json_field->>'some_attr') )

This could perhaps be a new option on the Index class:

class SomeTable(models.Model):
    some_json_field = models.JSONField()

    class Meta:
        indexes = (
            models.Index(models.RawSQL("some_json_field->>'some_attr'", params=[]), name="unique_attr", unique=True),
        )

[Mariusz Felisiak]

Django 4.0 will support functional unique constraints which are implemented internally by unique indexes, see #30916 and 3aa545281e0c0f9fac93753e3769df9e0334dbaa. I don't think there is a need for a new API.

Duplicate of #30916.