Re: Do not understand high estimates of index scan vs seq scan

Hi all,

I do not understand why postgreSQL estimates an index scan only half as fast as a seq scan:

system=# explain select * from queuelog;                            QUERY PLAN
-------------------------------------------------------------------
 Seq Scan on queuelog  (cost=0.00..20530.29 rows=610929 width=148)
(1 row)

system=# explain select * from queuelog where queuelog.start_time >= '2013-05-20 8:30' and queuelog.start_time <= '2013-06-21 17:0';
                                                                        QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using queuelog_start_time on queuelog  (cost=0.00..13393.18 rows=316090 width=148)
   Index Cond: ((start_time >= '2013-05-20 08:30:00+02'::timestamp with time zone) AND (start_time <= '2013-06-21 17:00:00+02'::timestamp with time zone))


Is that solely because it nees to compare each index value to a fixed date/time?
I would assume the index would be much smaller then the actual data, or is it only based on the amount of rows?