Re: Another oddity in handling of WCO constraints in postgres_fdw

On 2017/10/04 21:28, Ashutosh Bapat wrote:
> On Wed, Oct 4, 2017 at 5:32 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Wed, Oct 4, 2017 at 6:40 AM, Ashutosh Bapat
>> <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
>>> We can
>>> check whether a row being sent from the local server to the foreign
>>> server obeys WCO, but what foreign server does to that row is beyond
>>> local server's scope.
>>
>> But I think right now we're not checking the row being sent from the
>> local server, either.

We don't check the row *before* sending it to the remote server, but
check the row returned by ExecForeignInsert/ExecForeignUpdate, which is
allowed to have been changed by the remote server. In postgres_fdw, we
currently return the data actually inserted/updated if RETURNING/AFTER
TRIGGER present, but not if WCO only presents. So, for the postgres_fdw
foreign table, WCO is enforced on the data that was actually
inserted/updated if RETURNING/AFTER TRIGGER present and on the original
data core supplied if WCO only presents, which is inconsistent behavior.

> Didn't 7086be6e3627c1ad797e32ebbdd232905b5f577f fix that?

No. The commit addressed another issue.

>> The WCO that is being ignored isn't a
>> constraint on the foreign table; it's a constraint on a view which
>> happens to reference the foreign table. It seems quite odd for the
>> "assume constraints are valid" property of the foreign table to
>> propagate back up into the view that references it.

Agreed.

> The view with WCO is local but the modification which violates WCO is
> being made on remote server by a trigger on remote table. Trying to
> control that doesn't seem to be a good idea, just like we can't
> control what rows get inserted on the foreign server when they violate
> local constraints. I am using local constraints as an example of
> precedence where we ignore what's happening on remote side and enforce
> whatever we could enforce locally. Local server should make sure that
> any rows sent from local server to the remote server do not violate
> any local WCO.

Seems odd (and too restrictive) to me too.

Best regards,
Etsuro Fujita