POSTGRESQL :INSERT INTO ... SELECT with auto generated column

Question

I have two tables A(id, col1, col2) and B(col3, col4, col5, col6)

the column "id" in table A is an auto generated, not null, primary key.

To insert data from table B to table A, I am trying

INSERT INTO A(col1, col2)
(SELECT col3, col4 from B)

This statement throws error

ERROR:  null value in column "id" violates not-null constraint
DETAIL: Failing row contains (null, "abc", "def")

I also tried

INSERT INTO A(id, col1, col2)
(SELECT DEFAULT, col3, col4 from B)

this throws error

ERROR:  syntax error at or near "DEFAULT"

Why is postgres not auto generating value for column "id" ? The query works if I provide "id" from table B, or if I insert single row (without select) and providing "DEFAULT" keyword for auto generated column.

EDIT: Table creation query

CREATE TABLE A 
(
id bigint NOT NULL, 
col1 character varying(10), 
col2 character varying(10), 
CONSTRAINT A_pk PRIMARY KEY (id) 
); 
ALTER TABLE A OWNER TO user1; 

CREATE SEQUENCE A_id_seq 
START WITH 1 
INCREMENT BY 1 
NO MINVALUE 
NO MAXVALUE 
CACHE 1; 

ALTER TABLE A_id_seq OWNER TO user1; 

ALTER SEQUENCE A_id_seq OWNED BY A.id; 

Answer 1

Don't use an explicit sequence. Just define the column as serial or bigserial:

CREATE TABLE A (
    id bigserial NOT NULL, 
    col1 character varying(10), 
    col2 character varying(10), 
    CONSTRAINT A_pk PRIMARY KEY (id) 
); 

Then the insert will be fine. Here is a SQL Fiddle.