From: | Michał Kłeczek <michal(at)kleczek(dot)org> |
---|---|
To: | PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Invalid query generated by postgres_fdw with UNION ALL and ORDER BY |
Date: | 2024-03-07 06:08:40 |
Message-ID: | 0714C8B8-8D82-4ABB-9F8D-A0C3657E7B6E@kleczek.org |
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Lists: | pgsql-hackers |
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
—
Kind regards,
Michal
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