Need help on index!!!

Hi guru

I have a table with 300,000 records. I also created a index on it. But
the postgresql use sequence scan every time, when i search one record.
It is so slow. why?

here is create table script
create table pmmeasure (dataid bigint not null,
measurenum bigint DEFAULT nextval('measurenum')
UNIQUE NOT NULL,
name varchar(100) not null,
value varchar(100) not null,
constraint pk_pmmeasure primary key (measurenum),
constraint fk_pmmeasure_1 foreign key (dataid)
references pmdata (dataid)
);
create index index_pm on pmmeasure (dataid);

When I do like
explain analyze select * from pmmeasure where dataid = 10000;

it shows that postgresql always sequence scan for that record.

Thanks a lot.

Johnson