performance index scan vs bitmap-seq scan. - Mailing list pgsql-performance

The server is running 8.2.5 FreeBSD 6.1 with 3 GB of RAM.
I have a table with over 100M rows. I have a unique index (primary key) on column name called aid.
The select count(aid) .. does a Bitmap heap scan when the right side condition is above 100,000,000 (if i take one zero off it does a pure index scan).
My question : why is the optimizer choosing an Bitmap Heap Scan when count can be done with index.

When i set the bitmap scan to off, it selects an seq scan, after which i turn the seq scan off , then it does index scan only but it takes 8 minutes longer than doing a index scan+bitmap scan.

Any insight is appreciated.
thank you !
g





explain select count(aid) from topcat.aid where aid >= 10000000 and aid <= 100000000;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Aggregate  (cost=2786143.42..2786143.43 rows=1 width=4)
   ->  Bitmap Heap Scan on aid  (cost=142949.13..2742507.95 rows=17454188 width=4)
         Recheck Cond: ((aid >= 10000000) AND (aid <= 100000000))
         ->  Bitmap Index Scan on idx_aid_aid  (cost=0.00..142949.13 rows=17454188 width=0)
               Index Cond: ((aid >= 10000000) AND (aid <= 100000000))
(5 rows)

 explain select count(aid) from topcat.aid where aid >= 100000 and aid <= 100000000;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Aggregate  (cost=2786143.42..2786143.43 rows=1 width=4)
   ->  Bitmap Heap Scan on aid  (cost=142949.13..2742507.95 rows=17454188 width=4)
         Recheck Cond: ((aid >= 100000) AND (aid <= 100000000))
         ->  Bitmap Index Scan on idx_aid_aid  (cost=0.00..142949.13 rows=17454188 width=0)
               Index Cond: ((aid >= 100000) AND (aid <= 100000000))
(5 rows)

 explain select count(aid) from topcat.aid where aid >= 1000000 and aid <= 100000000;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Aggregate  (cost= 2786143.42..2786143.43 rows=1 width=4)
   ->  Bitmap Heap Scan on aid  (cost=142949.13..2742507.95 rows=17454188 width=4)
         Recheck Cond: ((aid >= 1000000) AND (aid <= 100000000))
         ->  Bitmap Index Scan on idx_aid_aid  (cost= 0.00..142949.13 rows=17454188 width=0)
               Index Cond: ((aid >= 1000000) AND (aid <= 100000000))
(5 rows)

explain select count(aid) from topcat.aid where aid >= 1000000 and aid <= 10000000;
                                 QUERY PLAN
-----------------------------------------------------------------------------
 Aggregate  (cost=5.58..5.59 rows=1 width=4)
   ->  Index Scan using idx_aid_aid on aid  (cost= 0.00..5.58 rows=1 width=4)
         Index Cond: ((aid >= 1000000) AND (aid <= 10000000))