seq-scan or index-scan

Dear list,

i have a table and i'm selecting all records without a where-condition, and i don't need a ORDER BY:

production=*# explain analyse select * from boxes;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------
Seq Scan on boxes (cost=0.00..990783.99 rows=6499 width=581) (actual time=6.514..4588.136 rows=3060 loops=1)
Total runtime: 4588.729 ms
(2 rows)

It's slow, so i tried with an ORDER BY:

production=# explain analyse select * from boxes order by id;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
Index Scan using boxes_pkey on boxes (cost=0.00..162437.00 rows=6499 width=581) (actual time=0.065..55.730 rows=3060 loops=1)
Total runtime: 56.169 ms
(2 rows)

Why not using the index (it's a primary key) for the first query?

PG 9.1.3

Andreas
--
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