Thanks @mrjn, but I’m not sure we’re referring to the same thing. The @urnary predicate seems it would apply to a Node → Predicate → Node relationship where there can be only one such relationship per origin node. But for the “unique index”, I was thinking:
Node -> Predicate with UIX -> "Unique literal value"
Where there is a database-wide guarantee that for “Predicate with UIX”, one and only one node has a given “Unique literal value”.
That way if I have ten source systems all sending entities into dgraph and populating “Predicate with UIX” with hypothetically conflicting values, I have a uniqueness guarantee at the database level that can never be violated even if someone messes up by not checking first to see if that value already exists. Basically extending the enforced uniqueness that UID’s have to other predicates as well.