index usage makes problem

I had 50 lakh records in my table...

while counting that am using that row in where condition... which makes problem, cpu is waiting for device...

Debian OS, postresql 7.4, 50 lakh records.

Query is

EXPLAIN ANALYZE select count(call_id) from call_log where call_id > 1;

while seeing the top, cpu is waiting for i/o, and without this call_id condition if i do
       EXPLAIN ANALYZE select count(oid) from call_log where oid > 1;
it executed in 21 seconds....