I've run into an existing behavior where
xmax(), and various other system tables, return an error when included in the
RETURNING list on a partitioned table.
ERROR: cannot retrieve a system column in this context
`
This issue got a fair airing back in 2020:
AW: posgres 12 bug (partitioned table)
https://www.postgresql.org/message-id/flat/GVAP278MB006939B1D7DFDD650E383FBFEACE0%40GVAP278MB0069.CHEP278.PROD.OUTLOOK.COM#908f2604081699e7f41fa20d352e1b79I'm using 14.4, and just ran into this behavior today. I'm wondering if there has been any new work on this subject, or anything to take into account moving forward?
I'm not a C coder, and do not know the Postgres internals, but here's what I gleaned from the thread:
* Available system columns depend on the underlying table access method, and may/will vary across AMs. For example, the columns implemented by
heap is what the docs describe, an FDW could be anything, and Postgres has no control of what, if any, system column-like attributes they support, and future and hypothetical AMs may have different sets.
* Rather than return garbage results, or a default of 0, etc., the system throws the error I ran into.
I'd be happier working with a
NULL result than garbage, ambiguous results, or errors...but an error is the current behavior. Agreed on that, I'd rather an error than a bad/meaningless result. Postgres' consistent emphasis on correctness is easily one of its greatest qualities.
In my case, I'm upgrading a lot of existing code to try and capture a more complete profile of what an
UPSERT did. Right now, I grab a count(*) of the rows and return that. Works fine. A revised snippet looks a bit like this:
------------------------------------------------------------...UPSERT code
returning xmax as inserted_transaction_id),
status_data AS (
select count(*) FILTER (where inserted_transaction_id = 0) AS insert_count,
count(*) FILTER (where inserted_transaction_id != 0) AS estimated_update_count,
pg_current_xact_id_if_assigned()::text AS transaction_id
from inserted_rows),
...custom logging code
-- Final output/result.
select insert_count,
estimated_update_count,
transaction_id
from status_data;
------------------------------------------------------------
This fails on a partitioned table because xmax() may not exist. In fact, it does exist in all of those tables, but the system doesn't know how to guarantee that. I know which tables are partitioned, and can downgrade the result on partitioned tables to the count(*) I've been using to date. But now I'm wondering if working with xmax() like this is a poor idea going forward. I don't want to lean on a feature/behavior that's likely to change. For example, I noticed the other day that MERGE does not support RETURNING.
I'd appreciate any insight or advice you can offer.
