From: PG Bug reporting form <noreply@postgresql.org>
To: pgsql-bugs@lists.postgresql.org
Cc: michal@kleczek.org
Subject: BUG #18381: Invalid query generated by postgres_fdw with UNION ALL and ORDER BY
Date: Thu, 07 Mar 2024 10:22:33 +0000
Message-ID: <18381-137456acd168bf93@postgresql.org> (raw)
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
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Cc: noreply@postgresql.org, pgsql-bugs@lists.postgresql.org, michal@kleczek.org
Subject: Re: BUG #18381: Invalid query generated by postgres_fdw with UNION ALL and ORDER BY
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