Why tuning is not giving better performance?

Hi, all
Sorry to interrupt. I am using PostgreSQL 9.1.1 and populated database
with DBT-3.
I changed some parameters the default postgresql.conf, like
shared_buffers and checkpoint_segments. But the query is taking the
same time, using the same query plan. The output is same for
"explain".
Am I doing the right way to tune the database?

I'm using the following query, TPC-H Q1:
select
l_returnflag,
l_linestatus,
sum(l_quantity) as sum_qty,
sum(l_extendedprice) as sum_base_price,
sum(l_extendedprice * (1 - l_discount)) as sum_disc_price,
sum(l_extendedprice * (1 - l_discount) * (1 + l_tax)) as sum_charge,
avg(l_quantity) as avg_qty,
avg(l_extendedprice) as avg_price,
avg(l_discount) as avg_disc,
count(*) as count_order
from
lineitem
where
l_shipdate <= date('1998-12-01') - interval '90 days'
group by
l_returnflag,
l_linestatus
order by
l_returnflag,
l_linestatus;

I keep asking questions in here. Sorry for any interrupt.
-Yangyang Wu