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

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);

---

There is a quick and dirty way with superuser privileges:

UPDATE pg_attribute
SET    attnotnull = FALSE
WHERE  attrelid = 'tbl_b'::regclass  -- schema-qualify if needed!
AND    attnotnull
AND    NOT attisdropped
AND    attnum > 0;

The shortcut is tempting. But if you screw this up you may end up breaking your system. The basic rule is: never tamper with system catalogs directly.

The clean way only needs regular privileges to alter the table: automate it with dynamic SQL in a DO statement (this implements what Denis already suggested):

DO
$$
BEGIN

EXECUTE (
   SELECT 'ALTER TABLE tbl_b ALTER '
       || string_agg (quote_ident(attname), ' DROP NOT NULL, ALTER ')
       || ' DROP NOT NULL'
   FROM   pg_catalog.pg_attribute
   WHERE  attrelid = 'tbl_b'::regclass
   AND    attnotnull
   AND    NOT attisdropped
   AND    attnum > 0
   );

END
$$

Still very fast. Execute care with dynamic commands and be wary of SQL injection.

This is a spin-off from this bigger answer:
Generate DEFAULT values in a CTE UPSERT using PostgreSQL 9.3

There we have the need to drop NOT NULL constraints from a table created with:

CREATE TABLE tbl_b (LIKE tbl_a INCLUDING DEFAULTS);

Since, per documentation:

Not-null constraints are always copied to the new table.

---

I had a scenario needing to remove the NOT NULL from every field with a certain name across the entire database. Here was my solution. The where clause could be modified to handle whatever search pattern you need.

DO $$ DECLARE row record;
BEGIN FOR row IN 
    (
        SELECT
            table_schema, table_name, column_name
        FROM
            information_schema.columns 
        WHERE
            column_name IN ( 'field1', 'field2' )
    )
    LOOP
        EXECUTE 
          'ALTER TABLE ' || row.table_schema || '.' || row.table_name || ' ALTER '
       || string_agg (quote_ident(row.column_name), ' DROP NOT NULL, ALTER ')
       || ' DROP NOT NULL;';
    END LOOP;
END; $$;

piggybacked off some other examples, this worked better for my needs

---

Yes, it is. I had the same issue..

To resolve, I had to write a C# .net script which traversed all the plSql database and removed all the matching constraints..

For, specific info on how to remove single constraints, pl follow the link. http://www.techonthenet.com/oracle/foreign_keys/drop.php

---