Bitmap Heap Scan and Bitmap Index Scan

[Arup Rakshit]

Here is a explain plan of a very simple query:

aruprakshit=# explain analyze select first_name, last_name from users where 
lower(state) = 'colorado';
                                                            QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on users  (cost=5.86..161.40 rows=203 width=13) (actual 
time=0.134..0.444 rows=203 loops=1)
   Recheck Cond: (lower((state)::text) = 'colorado'::text)
   Heap Blocks: exact=106
   ->  Bitmap Index Scan on lower_state_users_idx  (cost=0.00..5.81 rows=203 
width=0) (actual time=0.098..0.098 rows=203 loops=1)
         Index Cond: (lower((state)::text) = 'colorado'::text)
 Planning time: 0.263 ms
 Execution time: 0.517 ms
(7 rows)

I read this 
https://www.postgresql.org/message-id/12553.1135634231%40sss.pgh.pa.us 
<https://www.postgresql.org/message-id/12553.1135634...@sss.pgh.pa.us> and 
https://www.postgresql.org/message-id/464F3C5D.2000700%40enterprisedb.com 
<https://www.postgresql.org/message-id/464f3c5d.2000...@enterprisedb.com>  to 
understand what this bitmap heap scan and index scan is. But there are some 
questions still in mind which I am not able to figure out yet.

Does bitmap index apply when normal index scan is costly?
Does bitmap index always store page number of matching tuples instead of just 
the tuples?
What is Heap Blocks: exact=106 ?
Why the cost is higher in Heap scan than index scan?

Thanks,

Arup Rakshit
a...@zeit.io