@index(unique: true) and historical should have exclusion constraints set.

[stwiname]

Description

This is a follow up issue to https://github.com/subquery/subql/issues/1518. When historical is enabled and @index(unique: true) is set on an entity field a GiST index is used with _block_height. But GiST doesn't support unique indexes so the fields on this entity will not be unique.

We should set exclusion constraints on the table to enforce the uniqueness

[ianhe8x]

I think unique index doesn't offer more efficiency than a normal index, so we can safely say it is for fool-proofing. In that case, we can ignore unique:true or refuse it. I'm worried more exclusion constraints will have performance impact.