Re: multi column foreign key for implicitly unique columns

Markus Bertheau wrote:
> Hi,
> 
> PostgreSQL doesn't allow the creation of a foreign key to a combination
> of fields that has got no dedicated unique key but is unique nonetheless
> because a subset of the combination of fields has a unique constraint.
[snip example]
> Is this on purpose? I think the foreign key should be allowed. Creating
> an extra unique key only has a negative impact on performance, right?

As you say, the uniqueness is guaranteed so there's no good reason why 
it couldn't be made to work. It's probably more of an implementation 
issue. Unique constraints are implemented with an index, so I'm guessing 
the FK code assumes there is an index there to check against.

--   Richard Huxton  Archonet Ltd