Hi Venkatesh,
you're right about the current limitations of constraint support.
Currently, this is the state of affairs and I don't know of any planned improvements on this.
Once thing though: the existence of a unique index does not lead to an inferred unique constraint in any database we support.
To express semantically correct that there should be a unique column set, please do use the ADD CONSTRAINT <constraint_name> UNIQUE command.
This will in turn create a unique index (and no, you can't just reuse the existing unique index...).
BTW: nicely posted request - I can just take your example code and run it. That's how it should actually be done!
- Lars