This might be a fairly interesting project for someone who would like to learn about how access methods in PostgreSQL work. One downside is that the project wouldn't address some of the more fundamental problems with the current hash index implementation, so in a sense it is rearranging the deckchairs on the Titanic -- but it would be a fun project to hack on.

Basically, just add UNIQUE support for hash indexes. That just means walking through the chain of pages in a given hash bucket when doing an insertion, to verify that none of the existing members of the bucket are equal to the newly-inserted item. One consideration is how this would interact with the existing hash index locking protocol.

Neil C. actually implemented this functionality aeons ago, but never got the patch into shape for application to the tree. You could use that patch (linked below), or start from scratch.

Related links:

• Version of the patch by Neil C.
  

Interested hackers: