PERFORMANCE and SIZE

Hello,

I'm a new PostgresSql user and I do not know so much about the
performance mechanisms currently implemented and available.

So, as a dummy user I think that something strange is happening with me.
When I run the following command:

explain analyze select * from customer
where c_last = 'ROUGHTATION' and
c_w_id = 1 and
c_d_id = 1
order by c_w_id, c_d_id, c_last, c_first limit 1;

I receive the following results:

(Customer table with 60.000 rows) -
QUERY PLAN
---------------------------------------------------------------------------
-----------------------------------------------------------
Limit (cost=4.84..4.84 rows=1 width=283) (actual time=213.13..213.13
rows=0 loops=1)
-> Sort (cost=4.84..4.84 rows=1 width=283) (actual
time=213.13..213.13 rows=0 loops=1)
Sort Key: c_w_id, c_d_id, c_last, c_first
-> Index Scan using pk_customer on customer (cost=0.00..4.83
rows=1 width=283) (actual time=211.93..211.93 rows=0 loops=1)
Index Cond: ((c_w_id = 1) AND (c_d_id = 1))
Filter: (c_last = 'ROUGHTATION'::bpchar)
Total runtime: 213.29 msec
(7 rows)

(Customer table with 360.000 rows) -
QUERY PLAN
---------------------------------------------------------------------------
-------------------------------------------------------------
Limit (cost=11100.99..11101.00 rows=1 width=638) (actual
time=20.82..20.82 rows=0 loops=1)
-> Sort (cost=11100.99..11101.00 rows=4 width=638) (actual
time=20.81..20.81 rows=0 loops=1)
Sort Key: c_w_id, c_d_id, c_last, c_first
-> Index Scan using pk_customer on customer
(cost=0.00..11100.95 rows=4 width=638) (actual time=20.40..20.40 rows=0
loops=1)
Index Cond: ((c_w_id = 1) AND (c_d_id = 1))
Filter: (c_last = 'ROUGHTATION'::bpchar)
Total runtime: 21.11 msec
(7 rows)

Increasing the number of rows the total runtime decreases.
The customer table has the following structure:
CREATE TABLE customer
(
c_id int NOT NULL ,
c_d_id int4 NOT NULL ,
c_w_id int4 NOT NULL ,
c_first char (16) NULL ,
c_middle char (2) NULL ,
c_last char (16) NULL ,
c_street_1 char (20) NULL ,
c_street_2 char (20) NULL ,
c_city char (20) NULL ,
c_state char (2) NULL ,
c_zip char (9) NULL ,
c_phone char (16) NULL ,
c_since timestamp NULL ,
c_credit char (2) NULL ,
c_credit_lim numeric(12, 2) NULL ,
c_discount numeric(4, 4) NULL ,
c_balance numeric(12, 2) NULL ,
c_ytd_payment numeric(12, 2) NULL ,
c_payment_cnt int4 NULL ,
c_delivery_cnt int4 NULL ,
c_data text NULL
);

ALTER TABLE customer ADD
CONSTRAINT PK_customer PRIMARY KEY
(
c_w_id,
c_d_id,
c_id
);

Does anybody know what is happening ?

Thanks !!!!

Alfranio Junior