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
Aleksander Alekseev <aleksander@timescale.com> 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.