Of course, some indirection would be needed to avoid the naive solution of simply duplicating values.
Maybe Unix inodes is the best analogy here. aj
I think this would be another powerful feature by making it so much
easier when faced with records/objects that can have multiple unique
keys where both are not always used. You wouldn't have to use secondary
indexes which really aren't suitable for high cardinality (high
uniqueness) indexes and better suited for range queries.
