From: | "Eric G(dot) Miller" <egm2(at)jps(dot)net> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Unique or Primary Key? |
Date: | 2001-05-03 01:50:09 |
Message-ID: | 20010502185009.D4206@calico.local |
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Thread: | |
Lists: | pgsql-general |
On Thu, May 03, 2001 at 12:58:03AM +0100, pgsql(at)itsbruce(dot)uklinux(dot)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(at)jps(dot)net>
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