Re: foreign key restrictions

"Richard Broersma" <richard(dot)broersma(at)gmail(dot)com> writes:

> On Sun, Aug 10, 2008 at 1:15 AM, <rafal(at)zorro(dot)isa-geek(dot)com> wrote:
>
>> Since table TWO has a unique constraint on column ID, the (ID, OFONE) pair
>> will also be unique, obviously.
>
> This statement is not completely true. The only part of the pair that
> is true is ID. Also there is not unique constraint on the pare. So
> there is no way to PG to build a foreing key on the pair.

Uhm, afaics he's right. if ID is unique not null then <ID, OFONE> also has to
be unique. That is, there could be duplicate values of OFONE but they'll all
have different values of ID anyways.

I'm not sure if there's a fundamental reason why there has to be an index that
exactly matches the foreign key or not -- offhand I can't think of one.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!