How SELECT statement can cause disk write IO - Query Plan 1

query_plan_1

                                                                QUERY PLAN                                                                 
-------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.87..51.28 rows=100 width=206) (actual time=0.023..24.739 rows=100 loops=1)
   Buffers: shared hit=255 read=149
   ->  Nested Loop  (cost=0.87..10080882.37 rows=20000000 width=206) (actual time=0.023..24.714 rows=100 loops=1)
         Buffers: shared hit=255 read=149
         ->  Index Scan using book_pkey on book  (cost=0.44..666418.44 rows=20000000 width=30) (actual time=0.012..0.069 rows=100 loops=1)
               Buffers: shared hit=4
         ->  Index Scan using author_pkey on author  (cost=0.43..0.47 rows=1 width=168) (actual time=0.245..0.245 rows=1 loops=100)
               Index Cond: (id = book.author_id)
               Buffers: shared hit=251 read=149
 Planning Time: 0.438 ms
 Execution Time: 24.794 ms
(11 rows)