Re: Unique or Primary Key?

On Thu, May 03, 2001 at 12:58:03AM +0100, pgsql@itsbruce.uklinux.net wrote:
> This table is man-in-the-middle of a many-to-many relationship:
>
> CREATE TABLE cv_entries (
>         subscriber INTEGER NOT NULL
>                 REFERENCES subscribers
>                         ON DELETE CASCADE
>                         ON UPDATE CASCADE,
>         entry_type INTEGER NOT NULL
>             REFERENCES cv_entry_types
>                         ON DELETE CASCADE
>                         ON UPDATE CASCADE,
>         ordinal INTEGER,
>     value1 TEXT,
>     value2 TEXT,
>     minimum_trust SMALLINT,
>     UNIQUE(subscriber, entry_type, ordinal)
> );
>
> I used a unique index here because I couldn't see any reason for a
> Primary Key - this table will always be searched on either the
> subscriber or entry_type index.
>
> Was I wrong?  Should this be a Primary Key?

I think it's a distinction without a difference. A primary key is just a
way to identify a unique tuple that's been chosen from a possible
set of candidate keys (often there's only one candidate).  And,
primary keys are enforced with a unique index...

--
Eric G. Miller <egm2@jps.net>