merge semi join cost calculation error - Mailing list pgsql-hackers

Merge Semi Join  (cost=82.97..580.24 rows=580 width=8) (actual time=0.503..9557.396 rows=721 loops=1)

  Merge Cond: (tips.users_id = follows.users_id_to)

  ->  Index Scan using tips_idx_users_id01 on tips  (cost=0.43..8378397.19 rows=2491358 width=16) (actual time=0.009..9231.585 rows=2353914 loops=1)

  ->  Sort  (cost=1.77..1.82 rows=22 width=8) (actual time=0.052..0.089 rows=28 loops=1)

        Sort Key: follows.users_id_to

        Sort Method: quicksort  Memory: 26kB

        ->  Seq Scan on follows  (cost=0.00..1.27 rows=22 width=8) (actual time=0.013..0.020 rows=28 loops=1)

              Filter: (users_id_from = 1)

He has PostgreSQL 10.5. I cannot to understand to too low total cost of Merge Semi Join because subnode has very high cost 8378397.