CREATE TABLE t (
  refid                 BIGINT NOT NULL,
  created               TIMESTAMPTZ NOT NULL
);
CREATE INDEX t_refid_created ON t (refid, created);

-- index only scan t_refid_created_desc_idx
SELECT DISTINCT ON (refid) * FROM t
ORDER BY refid, created DESC;

-- index scan t_refid_created_idx
SELECT refid, max(created) FROM t GROUP BY refid;

CREATE index t_refid_created_desc_idx ON t (refid, created DESC);

Unique  (cost=0.56..850119.78 rows=291 width=16)
  ->  Index Only Scan using t_refid_created_desc_idx on t  (cost=0.56..808518.47 rows=16640527 width=16)

Finalize GroupAggregate  (cost=599925.13..599932.41 rows=291 width=16)
  Group Key: refid
  ->  Sort  (cost=599925.13..599926.59 rows=582 width=16)
        Sort Key: refid
        ->  Gather  (cost=599837.29..599898.40 rows=582 width=16)
              Workers Planned: 2
              ->  Partial HashAggregate  (cost=598837.29..598840.20 rows=291 width=16)
                    Group Key: refid
                    ->  Parallel Seq Scan on t  (cost=0.00..564169.53 rows=6933553 width=16)