pg_catalog.pg_get_serial_sequence() returns NULL

Hi!

I use PG 8.1.5

I execute in psql next comands:

create table t_table (gid serial, name varchar);

select pg_catalog.pg_get_serial_sequence('t_table', 'gid');
 pg_get_serial_sequence
------------------------
 public.t_table_gid_seq

create table t_table_1() inherits (t_table);

\d t_table_1
                             Table "public.t_table_1"
 column |       Type        |                     Модификаторы
---------+-------------------+-------------------------------------------------------
 gid     | integer           | not null default nextval('t_table_gid_seq'::regclass)
 name    | character varying |
Inherit: t_table

select pg_catalog.pg_get_serial_sequence('t_table_1', 'gid');
 pg_get_serial_sequence
------------------------
<NULL>


How I can get name of sequence used by gid column of partition table? And why for t_table_1 pg_get_serial_sequence() returns NULL?
It returns NULL also if I create new sequence manually and use ALTER TABLE ALTER COLUMN <column> SET DEFAULT nextval(<my_new_sequence>::regclass).

In http://www.postgresql.org/docs/8.1/static/datatype.html#DATATYPE-SERIAL I can read that type SERIAL is equivalent to 

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
    colnameinteger DEFAULT nextval('tablename_colname_seq') NOT NULL
);

Thanks in advance

Sergey Karin