union vs union all

gistfile1.txt

[busfor_prod] # explain analyze SELECT * FROM cities where id in (select from_id FROM timetable_segments UNION ALL SELECT to_id FROM timetable_segments);
                                                                         QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=747.24..1988.45 rows=19318 width=442) (actual time=12.208..13.778 rows=606 loops=1)
   ->  HashAggregate  (cost=746.95..748.95 rows=200 width=4) (actual time=12.194..12.311 rows=606 loops=1)
         Group Key: timetable_segments.from_id
         ->  Append  (cost=0.00..665.16 rows=32716 width=4) (actual time=0.012..7.302 rows=32716 loops=1)
               ->  Seq Scan on timetable_segments  (cost=0.00..332.58 rows=16358 width=4) (actual time=0.012..2.133 rows=16358 loops=1)
               ->  Seq Scan on timetable_segments timetable_segments_1  (cost=0.00..332.58 rows=16358 width=4) (actual time=0.002..1.988 rows=16358 loops=1)
   ->  Index Scan using cities_pkey on cities  (cost=0.29..6.19 rows=1 width=442) (actual time=0.002..0.002 rows=1 loops=606)
         Index Cond: (id = timetable_segments.from_id)
 Planning time: 0.447 ms
 Execution time: 13.899 ms
(10 rows)

Time: 14.907 ms
[busfor_prod] # explain analyze SELECT * FROM cities where id in (select from_id FROM timetable_segments UNION SELECT to_id FROM timetable_segments);
                                                                            QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=2137.38..4842.78 rows=19318 width=442) (actual time=12.560..20.761 rows=606 loops=1)
   Hash Cond: (cities.id = timetable_segments.from_id)
   ->  Seq Scan on cities  (cost=0.00..2233.36 rows=38636 width=442) (actual time=0.007..4.401 rows=38636 loops=1)
   ->  Hash  (cost=1728.43..1728.43 rows=32716 width=4) (actual time=12.520..12.520 rows=606 loops=1)
         Buckets: 32768  Batches: 1  Memory Usage: 278kB
         ->  HashAggregate  (cost=1074.11..1401.27 rows=32716 width=4) (actual time=12.311..12.394 rows=606 loops=1)
               Group Key: timetable_segments.from_id
               ->  Append  (cost=0.00..992.32 rows=32716 width=4) (actual time=0.006..7.309 rows=32716 loops=1)
                     ->  Seq Scan on timetable_segments  (cost=0.00..332.58 rows=16358 width=4) (actual time=0.006..2.217 rows=16358 loops=1)
                     ->  Seq Scan on timetable_segments timetable_segments_1  (cost=0.00..332.58 rows=16358 width=4) (actual time=0.002..1.976 rows=16358 loops=1)
 Planning time: 0.226 ms
 Execution time: 20.903 ms
(12 rows)