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).