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 and https://www.postgresql.org/message-id/464F3C5D.2000700%40enterprisedb.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? |
