How to drop all NOT NULL constraints from a PostgreSQL table in one go

Is it possible to drop all NOT NULL constraints from a table in one go?

I have a big table with a lot of NOT NULL constraints and I'm searching for a solution that is faster than dropping them separately.

You can group them all in the same alter statement:

alter table tbl alter col1 drop not null,
                alter col2 drop not null,
                …

You can also retrieve the list of relevant columns from the catalog, if you feel like writing a do block to generate the needed sql. For instance, something like:

select a.attname
  from pg_catalog.pg_attribute a
 where attrelid = 'tbl'::regclass
   and a.attnum > 0
   and not a.attisdropped
   and a.attnotnull;

(Note that this will include the primary key-related fields too, so you'll want to filter those out.)

If you do this, don't forget to use quote_ident() in the event you ever need to deal with potentially weird characters in column names.

ALTER TABLE table_name ALTER COLUMN [SET NOT NULL| DROP NOT NULL]

If you want to drop all NOT NULL constraints in PostreSQL you can use this function:

CREATE OR REPLACE FUNCTION dropNull(varchar) RETURNS integer AS $$
DECLARE
  columnName varchar(50);
BEGIN

    FOR columnName IN  

select a.attname
  from pg_catalog.pg_attribute a
 where attrelid = $1::regclass
   and a.attnum > 0
   and not a.attisdropped
   and a.attnotnull and a.attname not in(

   SELECT               
  pg_attribute.attname
FROM pg_index, pg_class, pg_attribute 
WHERE 
  pg_class.oid = $1::regclass AND
  indrelid = pg_class.oid AND
  pg_attribute.attrelid = pg_class.oid AND 
  pg_attribute.attnum = any(pg_index.indkey)
  AND indisprimary)

          LOOP
          EXECUTE 'ALTER TABLE ' || $1 ||' ALTER COLUMN '||columnName||' DROP NOT NULL';        
        END LOOP;
    RAISE NOTICE 'Done removing the NOT NULL Constraints for TABLE: %', $1;
    RETURN 1;
END;
$$ LANGUAGE plpgsql;

Please note that the primary keys will be excluded.

Then you can call it using:

SELECT dropNull(TABLENAME);