Re: BUG #18295: In PostgreSQL a unique index on targeted columns is sufficient to support a foreign key - Mailing list pgsql-bugs

----- Mail original -----
> De: "gparc" <gparc@free.fr>
> À: "Laurenz Albe" <laurenz.albe@cybertec.at>
> Cc: "pgsql-bugs" <pgsql-bugs@lists.postgresql.org>
> Envoyé: Mercredi 24 Janvier 2024 17:01:04
> Objet: Re: BUG #18295: In PostgreSQL a unique index on targeted columns is sufficient to support a foreign key

> ----- Mail original -----
> De: "Laurenz Albe" <laurenz.albe@cybertec.at>
> À: "gparc" <gparc@free.fr>, "pgsql-bugs" <pgsql-bugs@lists.postgresql.org>
> Envoyé: Mercredi 24 Janvier 2024 16:28:45
> Objet: Re: BUG #18295: In PostgreSQL a unique index on targeted columns is
> sufficient to support a foreign key
>
> On Wed, 2024-01-24 at 11:11 +0100, gparc@free.fr wrote:
>> coming from Oracle, I'm surprised to see that in PostgreSQL, a foreign key
>> can be linked to a unique index
>> on the target table and not exclusively to a primary key constraint or
>> UNIQUE constraint.
>>
>> Is it a bug or an intended feature ? If the latter, I think the doc should
>> be amended to remove any ambiguity.
>
> Let's say it is an extension of the standard, but I cannot say if that is
> intended or not.  At any rate, it has been like that for a very long time,
> and changing it might make some users unhappy.
>
> There is some added value, in that you could reference a unique index
> that has an INCLUDE clause:
>
>  CREATE TABLE parent (id integer, payload integer, other integer);
>
>  CREATE UNIQUE INDEX ON parent (id) INCLUDE (payload);
>
>  CREATE TABLE child (id integer REFERENCES parent (id));
>
> So it might well be seen as a feature.
>
> Looking at the source, the function comment suggests that that undocumented
> feature may be there by accident:
>
> /*
> * transformFkeyCheckAttrs -
> *
> *  Make sure that the attributes of a referenced table belong to a unique
> *  (or primary key) constraint.  Return the OID of the index supporting
> *  the constraint, as well as the opclasses associated with the index
> *  columns.
> */
>
> The comment is speaking about a constraint, not a unique index.
>
> So perhaps the comment should be updated, along with a note in the documentation
> (in ddl.html and ref/create_table.sgml).
>
>
>> P.S. by the way, I don't know what the SQL standard states about that.
>
> That is simple: since the standard doesn't know indexes, it can only talk
> about referencing a constraint.
>
> Yours,
> Laurenz Albe
>
>
> Thanks Laurenz for your detailed reply.
> I agree also for an update of the documentation and source code.
>
> Concerning, the documentation, I propose to modify in
> https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-FK
> the following sentence :
> "A foreign key must reference columns that either are a primary key or form a
> unique constraint.
> This means that the referenced columns always have an index (the one underlying
> the primary key or unique constraint);"
> by
> "A foreign key must reference columns that either are a primary key or form a
> unique constraint or are specified in a unique index.
> This means that the referenced columns are always backed by a UNIQUE index."
>
> Regards
> Gilles

Is this new wording OK for you ?

Regards
Gilles