Subject: multi column foreign key for implicitly unique columns
From: Markus Bertheau <twanger@bluetwanger.de>
To: pgsql-sql@postgresql.org
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Message-Id: <1092734724.2627.4.camel@dicaprio.akademie1.de>
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Date: Tue, 17 Aug 2004 11:25:24 +0200
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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@bluetwanger.de>