performance index scan vs bitmap-seq scan.

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..142949.13rows=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..142949.13rows=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..142949.13rows=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))