From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
Cc: | David Pradier <david(dot)pradier(at)clarisys(dot)fr>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Is a primary key made of a couple columns so much better than a unique() constraint ? |
Date: | 2005-10-10 15:22:11 |
Message-ID: | 21047.1128957731@sss.pgh.pa.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-general |
Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> On Mon, Oct 10, 2005 at 04:44:54PM +0200, David Pradier wrote:
>> i'd like to know if it is better to use a primary key made of a couple
>> columns, than to use a constraint UNIQUE() on this couple columns,
>> regarding the sake of postgresql.
> In PostgreSQL, both primary keys and UNIQUE constraints are implemented
> via UNIQUE indexes, ergo there is no difference...
Just for the sake of completeness, there are exactly two differences:
* PRIMARY KEY implies NOT NULL on the key columns; UNIQUE doesn't.
* PRIMARY KEY creates a default target for foreign key references,
ie, if you've declared a primary key then you can later just say
"REFERENCES mytab" instead of spelling out "REFERENCES mytab(keycol)".
So "UNIQUE + NOT NULL" is pretty dang close to the same as "PRIMARY
KEY", but not quite.
regards, tom lane
From | Date | Subject | |
---|---|---|---|
Next Message | David Pradier | 2005-10-10 15:30:26 | Re: Is a primary key made of a couple columns so much better than a unique() constraint ? |
Previous Message | Michael Fuhr | 2005-10-10 15:20:47 | Re: PostgreSQL 8.1 vs. MySQL 5.0? |