[SQL] About Index

[Najm Hashmi]

Hi,
   I have created a table songs as follwoing:
CREATE TABLE  songs  (
     song_id  serial NOT NULL,
     title  character varying(50) NOT NULL,
    composer   varchar(50),
     performer  varchar(50),
     artist_id  int4 not null,
     file_size  int4,
     description  text,
     extrInfo  text,
     highmd5  text,
     lowmd5  text,
      PRIMARY KEY (song_id),
     constraint fk_songs_art foreign key( artist_id) references artist
);
I also created an other index on attribute title as:
create unique index title_idx on songs(title);

Then I vacuum analyzed  my data base.

On command line I type :
explain select title from songs where song_id between 1 and 15 order by
song_id;
NOTICE:  QUERY PLAN:

Sort  (cost=1.70..1.70 rows=14 width=16)
  ->  Seq Scan on songs  (cost=0.00..1.44 rows=14 width=16)

EXPLAIN

One thing I don't uderstand why query planer is using a seq scan on my
table.
Why it is not using indexing defined. Thank you all for all your help.

Regards.
Najm