Обсуждение: BUG #18381: Invalid query generated by postgres_fdw with UNION ALL and ORDER BY

The following bug has been logged on the website:

Bug reference:      18381
Logged by:          Michal Kleczek
Email address:      michal@kleczek.org
PostgreSQL version: 16.2
Operating system:   Linux
Description:

The following query:

SELECT * FROM (
  SELECT 2023 AS year, * FROM remote_table_1
  UNION ALL
  SELECT 2022 AS year, * FROM remote_table_2
)
ORDER BY year DESC;

yields the following remote query:

SELECT [columns] FROM remote_table_1 ORDER BY 2023 DESC

and subsequently fails remote execution.


Not really sure where the problem is - the planner or postgres_fdw.
I guess it is postgres_fdw not filtering out ordering keys.

This filtering would also be pretty useful in the following scenario (which
is also why I went through UNION ALL route and discovered this issue):

I have a table partitioned by year, partitions are remote tables.
On remote servers I have a GIST index that does not support ordering ([1])
so I would like to avoid sending ORDER BY year to remote servers.
Ideally redundant ordering should be filtered out.

[1]
https://www.postgresql.org/message-id/B2AC13F9-6655-4E27-BFD3-068844E5DC91%40kleczek.org

On Thu, 7 Mar 2024 at 23:35, PG Bug reporting form
<noreply@postgresql.org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference:      18381
> Logged by:          Michal Kleczek
> Email address:      michal@kleczek.org
> PostgreSQL version: 16.2
> Operating system:   Linux
> Description:

I see you've raised this on -hackers [1] already.   Since there's
already some discussion and a proposed fix there, I think it's better
the discussion continues there.

David

[1] https://postgr.es/m/0714C8B8-8D82-4ABB-9F8D-A0C3657E7B6E@kleczek.org