slow index scan performance

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Consider these 2 index scan produced by a query

->  Index Scan using response_log_by_activity on public.response_log rl2  (cost=0.00..51.53 rows=21 width=8) (actual time=9.017..9.056 rows=0 loops=34098)
                                       Output: rl2.activity_id, rl2.feed_id
                                       Index Cond: (rl2.activity_id = rl.activity_id)
                                       Filter: rl2.success
                                       Buffers: shared hit=3357159 read=153313
                           ->  Index Scan using activity_pkey on public.activity a  (cost=0.00..51.10 rows=1 width=12) (actual time=0.126..0.127 rows=1 loops=34088)
                                 Output: a.status_id, a.activity_id, a.visit_id
                                 Index Cond: (a.activity_id = rl.activity_id)
                                 Buffers: shared hit=137925 read=32728


And it's size

conscopy=# select pg_size_pretty(pg_relation_size('response_log_by_activity'::regclass));
 pg_size_pretty 
----------------
 7345 MB
(1 row)

conscopy=# select pg_size_pretty(pg_relation_size('activity_pkey'::regclass));
 pg_size_pretty 
----------------
 8110 MB
(1 row)

Index scan on response_log_by_activity is far slower. The table has just been repacked, and index rebuilt, but still slow.

Is there any other way to make it faster ?

Why Buffers: shared hit=3,357,159 read=153,313 on response_log_by_activity is much bigger than Buffers: shared hit=137925 read=32728 on activity_pkey while activity_pkey size is bigger ?

--
Regards,

Soni Maula Harriz

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