Subject: Re: MIN/MAX functions for a record

From: Tom Lane <[email-masked]>
Date: Fri, 22 Mar 2024 11:12:55 -0400

Aleksander Alekseev <[email-masked]> writes:
>> In my queries I often need to do MIN/MAX for tuples, for example:
>> SELECT MAX(row(year, month))
>> FROM (VALUES(2025, 1), (2024,2)) x(year, month);
>> This query throws:
>> ERROR: function max(record) does not exist
>> Was this ever discussed or is there something preventing the implementation?

> I believe it would be challenging to implement max(record) that would
> work reasonably well in a general case.

As long as you define it as "works the same way record comparison
does", ie base it on record_cmp(), I don't think it would be much
more than a finger exercise [*].  And why would you want it to act
any differently from record_cmp()?  Those semantics have been
established for a long time.

			regards, tom lane

[*] Although conceivably there are some challenges in getting
record_cmp's caching logic to work in the context of an aggregate.

---

From: Viliam Ďurina <[email-masked]>
Date: Fri, 22 Mar 2024 16:50:01 +0100

Exactly Tom, I see no fundamental problem for it not to be implemented,
since comparison operator is already implemented. In fact, MIN/MAX should
work for all types for which comparison operator is defined.

Regarding index support, there should not be an issue if the index is
defined for the record (e.g. `CREATE INDEX ON my_table(ROW(field_a,
field_b))`). However such indexes seem not to be supported. Whether a
composite index is compatible with a record created on the indexed fields
in every edge case I'm not sure...

Alexander, rewriting the year-month example is easy, but how would you
rewrite this query?

CREATE TABLE events(event_time TIMESTAMP, message VARCHAR, user_id VARCHAR);

You want a newest message for each user. It's easy with MAX(record):

SELECT user_id, MAX(ROW(event_time, message)).message
FROM events
GROUP BY user_id;

One option is to rewrite to a subquery with LIMIT 1

SELECT user_id, (SELECT message FROM events e2 WHERE e1.user_id=e2.user_id
ORDER BY event_time DESC LIMIT 1)
FROM events e1
GROUP BY user_id;

If your query already has multiple levels of grouping, multiple joins,
UNIONs etc., it gets much more complex. I also wonder if the optimizer
would pick the same plan as it would be if the MAX(record) is supported.

Viliam