Обсуждение: referencing system catalogs

Can someone explain the reasoning behind not allowing
a user table to contain a foreign key of a system table?

For example:

create table myusers (
    login    text    references pg_catalog.pg_shadow(usename),
    ...
);

Fails with a message about using system catalogs.
Using pg_user fails because it is a view and not a table.

Is it because updates to the catalogs can be done outside
of transactions?  Is that a good enough reason?

thanks,

elein
============================================================
elein@varlena.com        Varlena, LLC        www.varlena.com

          PostgreSQL Consulting, Support & Training

PostgreSQL General Bits   http://www.varlena.com/GeneralBits/
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I have always depended on the [QA] of strangers.

On Sat, 26 Jun 2004, elein wrote:

> Can someone explain the reasoning behind not allowing
> a user table to contain a foreign key of a system table?
>
> For example:
>
> create table myusers (
>     login    text    references pg_catalog.pg_shadow(usename),
>     ...
> );
>
> Fails with a message about using system catalogs.
> Using pg_user fails because it is a view and not a table.
>
> Is it because updates to the catalogs can be done outside
> of transactions?  Is that a good enough reason?

I believe it's because changes to at least some of the system catalogs can
be done without going through the entire sequence that an
update/insert/delete statement would (ie, the triggers might not be
fired thus meaning the constraint isn't properly checked).