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  1. postgres_fdw: Avoid "cursor can only scan forward" error.

  1. d844cd75a and postgres_fdw

    Devrim GÜNDÜZ <devrim@gunduz.org> — 2023-10-24T11:46:40Z

    Hi,

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?

Regards,

-- 
Devrim Gündüz
Open Source Solution Architect, PostgreSQL Major Contributor
Twitter: @DevrimGunduz , @DevrimGunduzTR

  2. Re: d844cd75a and postgres_fdw

    Etsuro Fujita <etsuro.fujita@gmail.com> — 2024-07-05T12:56:28Z

    On Tue, Oct 24, 2023 at 8:48 PM Devrim Gündüz <devrim@gunduz.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

  3. Re: d844cd75a and postgres_fdw

    Etsuro Fujita <etsuro.fujita@gmail.com> — 2024-07-19T05:01:54Z

    On Fri, Jul 5, 2024 at 9:56 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> On Tue, Oct 24, 2023 at 8:48 PM Devrim Gündüz <devrim@gunduz.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].

I pushed the fix and back-patched to v15.  Thanks for the report!

Best regards,
Etsuro Fujita

  4. Re: d844cd75a and postgres_fdw

    Devrim GÜNDÜZ <devrim@gunduz.org> — 2024-07-19T11:29:41Z

    Hi,

On Fri, 2024-07-19 at 14:01 +0900, Etsuro Fujita wrote:
> I pushed the fix and back-patched to v15.  Thanks for the report!

Thanks a lot!

Regards,
-- 
Devrim Gündüz
Open Source Solution Architect, PostgreSQL Major Contributor
Twitter: @DevrimGunduz , @DevrimGunduzTR