postgresql – Avoid sorting on the group indexed by caused by DATE_TRUNC

Is there a way to avoid Sort operation for GROUP BY, caused by the DATE_TRUNC function not recognized as tstamp, which is part of ix index? This function does not really affect the order maintained by the ix.

CREATE INDEX ix ON tbl(col, tstamp, num)

  SELECT col,
         DATE_TRUNC('MONTH', tstamp AT TIME ZONE 'UTC') AS trunc,
         SUM(num)
    FROM tbl
   WHERE col IN (?, ?, ...)
     AND tstamp >= ?
     AND tstamp <  ?
GROUP BY col, trunc

Precision setting for DATE_TRUNC is dynamic it could also be 'DAY' or 'YEAR'... so I cannot index this function without creating an index for each variation.

The execution plan that I get:

Finalize GroupAggregate
  Group Key: col, DATE_TRUNC('MONTH', tstamp AT TIME ZONE 'UTC')
  ->  Gather Merge
        Workers Planned: 2
        Workers Launched: 2
        ->  Partial GroupAggregate
              Group Key: col, DATE_TRUNC('MONTH', tstamp AT TIME ZONE 'UTC')
              ->  Sort
                    Sort Key: col, DATE_TRUNC('MONTH', tstamp AT TIME ZONE 'UTC')
                    Sort Method: external merge  Disk: ...kB
                    -> Parallel Index Only Scan using ix on tbl
                         Index Cond: ((col = ANY (...)) AND (tstamp >= ...) AND (tstamp < ...))
                         Heap Fetches: 0

The execution plan I want would look like this:

GroupAggregate
  Group Key: col, DATE_TRUNC('MONTH', tstamp AT TIME ZONE 'UTC')
  ->  Index Only Scan using ix on tbl
        Index Cond: ((col = ANY (...)) AND (tstamp >= ...) AND (tstamp < ...))
        Heap Fetches: 0

(I get it if I don't use it DATE_TRUNC)

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cordially
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