Re: Missing importing option of postgres_fdw

Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> writes:
> I agree with you on that point. And I think one option for that is that
> IMPORT FOREIGN SCHEMA only imports CHECK constraints of remote tables
> from a remote server that have convalidated = true. (If doing so, we
> wouldn't need to allow IMPORT FOREIGN SCHEMA to return ALTER FOREIGN
> TABLE statements.) But I'm not sure that that is a good idea. ISTM it
> would be better for IMPORT FOREIGN SCHEMA just to imports all CHECK
> constraints of remote tables, reflecting convalidated, and that we leave
> it to the user to do VALIDATE CONSTRAINT for locally defined constraints
> that have convalidated = false when matching constraints are validated
> on the remote server.

It would only be safe to automatically import CHECK constraints if they
contain no expressions that could evaluate differently on remote and local
--- IOW, only expressions that would be safe to transmit as remote query
conditions. I don't really think we should try to do that at all.

For starters, while it might seem like we could use is_foreign_expr()
on the conditions, there's no guarantee that the local server accurately
knows what functions on the foreign server are really safe. The "is safe
to transmit" condition isn't symmetric.

For that matter, there's no guarantee that we could even parse the
remote's constraint condition text without failing --- it might use SQL
features we don't have. (We have that risk already for DEFAULT
expressions, of course, but those tend to be a lot simpler.)

So I think worrying about convalidated is pretty pointless. Really,
it is up to the user to determine what constraints should be attached
to the foreign table, and IMPORT FOREIGN SCHEMA can't help much.

regards, tom lane