Re: Unique constraints for non-btree indexes

Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> I guess what you're talking about is a constrained index, of which a
> unique index is just a particular type. I suppose the actual constraint
> would be one of the operators defined for the operator class (since
> whatever the test is, it needs to be indexable). Although some would
> obviously be more useful than others...

I think the generalization that would be appropriate for GIST is that
a "unique" index guarantees there are no two entries x, y such that
x ~ y, where ~ is some boolean operator nominated by the opclass. We'd
probably have to insist that ~ is commutative (x ~ y iff y ~ x).

Concurrent insertion into a unique GIST index seems a bit nasty. In
btree we can identify a unique page to lock for any given key value
to ensure that no one else is concurrently inserting a conflicting
key, thus usually allowing concurrent insertions of different keys.
But I don't see how you do that for an arbitrary ~ operator.

regards, tom lane