Postgres 14

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CREATE TABLE tbl (
  id            serial PRIMARY KEY
, refreshed_utc timestamp
, purged_utc    timestamp
);

INSERT INTO tbl (refreshed_utc, purged_utc)
SELECT (timestamp '2022-1-1' - interval '6 month' * random())::timestamp
     , CASE WHEN random() > 0.99 THEN NULL ELSE LOCALTIMESTAMP END  -- much fewer NULL values overall!
FROM  generate_series (1, 100000);

CREATE INDEX tbl_foo_idx ON tbl (purged_utc, refreshed_utc);

CREATE TABLE

INSERT 0 100000

CREATE INDEX

VACUUM ANALYZE tbl;

VACUUM

-- added sort step (not as expected)
EXPLAIN (ANALYZE, BUFFERS)
SELECT *
FROM   tbl -- public.customer
WHERE  purged_utc IS NULL
AND    refreshed_utc < '2021-07-11 00:00:00'
ORDER  BY refreshed_utc
LIMIT  10;

QUERY PLAN

Limit  (cost=94.50..94.52 rows=10 width=20) (actual time=0.140..0.143 rows=10 loops=1)

  Buffers: shared hit=63 read=3 written=3

  ->  Sort  (cost=94.50..94.62 rows=50 width=20) (actual time=0.135..0.137 rows=10 loops=1)

        Sort Key: refreshed_utc

        Sort Method: top-N heapsort  Memory: 25kB

        Buffers: shared hit=63 read=3 written=3

        ->  Index Scan using tbl_foo_idx on tbl  (cost=0.42..93.42 rows=50 width=20) (actual time=0.061..0.118 rows=63 loops=1)

              Index Cond: ((purged_utc IS NULL) AND (refreshed_utc < '2021-07-11 00:00:00'::timestamp without time zone))

              Buffers: shared hit=63 read=3 written=3

Planning:

  Buffers: shared hit=34 read=1 written=1

Planning Time: 0.344 ms

Execution Time: 0.181 ms

EXPLAIN

-- no added sort step (as expected)
EXPLAIN (ANALYZE, BUFFERS)
SELECT *
FROM   tbl -- public.customer
WHERE  purged_utc = '2021-07-11 00:00:00'  -- not-null value
AND    refreshed_utc < '2021-07-11 00:00:00'
ORDER  BY refreshed_utc
LIMIT  10;

QUERY PLAN

Limit  (cost=0.42..8.44 rows=1 width=20) (actual time=0.041..0.041 rows=0 loops=1)

  Buffers: shared hit=1 read=2 written=2

  ->  Index Scan using tbl_foo_idx on tbl  (cost=0.42..8.44 rows=1 width=20) (actual time=0.040..0.040 rows=0 loops=1)

        Index Cond: ((purged_utc = '2021-07-11 00:00:00'::timestamp without time zone) AND (refreshed_utc < '2021-07-11 00:00:00'::timestamp without time zone))

        Buffers: shared hit=1 read=2 written=2

Planning Time: 0.070 ms

Execution Time: 0.056 ms

EXPLAIN