Re: d844cd75a and postgres_fdw

On Tue, Oct 24, 2023 at 8:48 PM Devrim Gündüz <devrim(at)gunduz(dot)org> wrote:
> I'm seeing an issue after upgrading from 12.13 to 15.4. This happens
> when we run a query against a foreign table (fdw on the same instance to
> a different database) -- but does not appear when we get rid of
> postgres_fdw:
>
> ERROR: cursor can only scan forward
> HINT: Declare it with SCROLL option to enable backward scan.
> CONTEXT: remote SQL command: MOVE BACKWARD ALL IN c1
>
> SQL state: 55000
>
> I attached the query. The name of the foreign table is
> "foobar.sys_user".
>
> Looks like the bug #17889, and this is the last email in that thread:
> https://www.postgresql.org/message-id/1852635.1682808624%40sss.pgh.pa.us
>
> OTOH, same query works (against the FDW) when we remove the following
> WHERE clause:
>
> WHERE
> tbl.table_status = 'A'
> AND tbl.table_id <> 1
> AND tbl.table_id <> - 2
>
> Any hints?

The error occurs when rescanning a postgres_fdw foreign relation, so I
think the reason why the query works would be that the planner chose a
join plan other than a nestloop join plan.

I proposed a fix for this in [1].

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAPmGK149UubRQGLH6QaBkhJvas%2BGz%2BT6tx2MBX9MTJpxDRKPBA%40mail.gmail.com