Thread: Is a primary key made of a couple columns so much better than a unique() constraint ?
Is a primary key made of a couple columns so much better than a unique() constraint ?
From
David Pradier
Date:
Hi everybody, 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. For example, i've got these two tables, 'buyer' and 'purchaser' and i want to make an association table buyer_purchaser in which i get only some couples (id_buyer, id_purchaser), with a unique() contraint on them. If i put a primary key on the couple, i won't use it, because the framework on top of it is unable to use a primary key made of a couple of columns. Si my question is : is it better, in so far as only the database is concerned, to have a primary key than a unique() contraint on a couple columns ? Thanks in advance for your answers, David -- David Pradier -- Directeur Technique de Clarisys Informatique -- Chef de projet logiciels libres / open-source
Re: Is a primary key made of a couple columns so much better than a unique() constraint ?
From
Martijn van Oosterhout
Date:
On Mon, Oct 10, 2005 at 04:44:54PM +0200, David Pradier wrote: > Hi everybody, > > 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... Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a > tool for doing 5% of the work and then sitting around waiting for someone > else to do the other 95% so you can sue them.
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Re: Is a primary key made of a couple columns so much better than a unique() constraint ?
From
David Pradier
Date:
> In PostgreSQL, both primary keys and UNIQUE constraints are implemented > via UNIQUE indexes, ergo there is no difference... Thanks Martijn, it's exactly what i wanted to know. Best regards, David -- David Pradier -- Directeur Technique de Clarisys Informatique -- Chef de projet logiciels libres / open-source
Re: Is a primary key made of a couple columns so much better than a unique() constraint ?
From
Tom Lane
Date:
Martijn van Oosterhout <kleptog@svana.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
Re: Is a primary key made of a couple columns so much better than a unique() constraint ?
From
David Pradier
Date:
> 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 Thanks too, Tom :-) Best regards, David -- David Pradier -- Directeur Technique de Clarisys Informatique -- Chef de projet logiciels libres / open-source