No merge sort?

I have a table "test" that looks like this:

CREATE TABLE test (
    id BIGINT,
    time INTEGER
);

There is an index:

CREATE INDEX idx ON test(id, time);

The table has been loaded with 2M rows, where time ranges sequentially
from 0 to 1999999 and id is random values from 0 to 49999.

This query:

SELECT * FROM idx WHERE id IN (...) AND time > 198000 AND time < 199800
ORDER BY time DESC LIMIT 20;

has an EXPLAIN ANALYZE of:

Limit  (cost=3635.28..3635.28 rows=20 width=12) (actual time=22.94..22.96 rows=14 loops=1)
  ->  Sort  (cost=3635.28..3635.28 rows=23 width=12) (actual time=22.93..22.93 rows=14 loops=1)
        ->  Index Scan using idx, idx, ..., idx, idx on test  (cost=0.00..3634.77 rows=23 width=12) (actual
time=1.01..22.10rows=14 loops=1) 
Total runtime: 29.12 msec

This query:

SELECT * FROM idx WHERE id IN (...) AND time < 199800 ORDER BY time DESC
LIMIT 20;

has an EXPLAIN ANALYZE of:

Limit  (cost=14516.46..14516.46 rows=20 width=12) (actual time=1448.83..1448.86 rows=20 loops=1)
  ->  Sort  (cost=14516.46..14516.46 rows=2527 width=12) (actual time=1448.82..1448.83 rows=21 loops=1)
        ->  Index Scan using idx, idx, ..., idx, idx on test  (cost=0.00..14373.67 rows=2527 width=12) (actual
time=0.14..1437.33rows=2048 loops=1) 
Total runtime: 1454.62 msec

Since the index will output 'time' sorted data for each 'id', why isn't
a merge sort being used here? A merge sort would reduce the execution
time back to 30 ms.

--
Taral <taral@taral.net>
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