int8 indices?

hi
my postgresql is:
depesz=# select version();
version
---------------------------------------------------------------
PostgreSQL 7.2devel on i686-pc-linux-gnu, compiled by GCC 3.0
(1 row)

(compiled from cvs 2 days ago).

int8 are working:
depesz=# select 2147483648::int4+1::int4;
ERROR: pg_atoi: error reading "2147483648": Numerical result out of range
depesz=# select 2147483648::int8+1::int8;
?column?
------------
2147483649

not i have table a:
depesz=# \d a
Table "a"
Column | Type | Modifiers
--------+--------+-----------
id | bigint |
Unique keys: aaa

depesz=# \d aaa
Index "aaa"
Column | Type
--------+--------
id | bigint
unique btree

depesz=# select count(*) from a;
count
-------
25981
(1 row)

(all of them are not nul:
depesz=# select count(*) from a where id is not null;
count
-------
25981
(1 row)
)

of course i vacuum'ed, but when i select anything from the table i get
seqscan:
depesz=# explain select * from a where id = 6970;
NOTICE: QUERY PLAN:

Seq Scan on a (cost=0.00..465.76 rows=1 width=8)

EXPLAIN

what could be possible reason?

depesz

--
hubert depesz lubaczewski http://www.depesz.pl/
------------------------------------------------------------------------
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and know the place for the first time. -- T. S. Eliot