multi column foreign key for implicitly unique columns

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.
Example:

CREATE TABLE p (
name TEXT PRIMARY KEY,
"type" TEXT
);

CREATE TABLE f (
name TEXT,
"type" TEXT,
FOREIGN KEY(name, "type") REFERENCES p(name, "type")
);
ERROR: there is no unique constraint matching given keys for referenced table "p"

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?

Thanks

--
Markus Bertheau <twanger(at)bluetwanger(dot)de>