Made Add/RemoveConstraint operations a noop for unique constraints with opclasses on SQLite.

[Hannes Ljungberg]

It currently recreates the table.

Related: https://code.djangoproject.com/ticket/32780

[Mariusz Felisiak]

I don't think we should make it a noop. On other databases (MySQL, Oracle) it creates a unique constraint without opclasses as ​documented: "opclasses are ignored for databases besides PostgreSQL" and IMO expected. opclasses are really PostgreSQL-specific and on other databases default operators are just fine.

[Simon Charette]

opclasses are really PostgreSQL-specific and on other databases default operators are just fine.

+1 to that, IMO Index.opclasses should be deprecated in favour of the PostgreSQL only OpClass expression.

[Hannes Ljungberg]

I don't think we should make it a noop. On other databases (MySQL, Oracle) it creates a unique constraint without opclasses

You're right. I'll close this ticket as the current behaviour on SQLite when opclasses are specified is to recreate the table with a unique constraint while ignoring opclasses instead of adding a unique index and this is probably the expected behaviour if expressions or conditions aren't used.

+1 to that, IMO Index.opclasses should be deprecated in favour of the PostgreSQL only OpClass expression.

Yup! Feels like we have a bunch of index-related stuff that should be deprecated. In addition to the opclasses argument; Meta.index_together, Meta.unique_together and possibly even Index.fields as we adapt Index.contains_expressions to ignore things that resolve to a Col.