nalbat=> create table t1 ( id integer primary key, val integer );
CREATE TABLE
nalbat=> \d t1
      Table "public.t1"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer | not null
 val    | integer |
Indexes:
    "t1_pkey" PRIMARY KEY, btree (id)

nalbat=> select column_name,is_nullable from information_schema.columns where table_name='t1';
 column_name | is_nullable
-------------+-------------
 id          | NO
 val         | YES
( 2  rows)