Want to fill the holes in a column previously defined with a serial in PostgreSQL

nextid.sql

-- Want to file the holes in a column previously defined with a serial
-- Useful only with small tables with less than 10k rows, over the operation
-- max() and the join will cost too much
--
CREATE OR REPLACE FUNCTION nextid()
RETURNS	integer
LANGUAGE plpgsql
AS $$
DECLARE	
	new_id integer;
BEGIN

WITH maxid AS (
  SELECT COALESCE(max(id),0) + 1 as maxid FROM boat
),

ids as (
  SELECT generate_series(1,maxid,1) as id FROM maxid
)

  SELECT min(ids.id) INTO new_id
  FROM boat
  RIGHT JOIN ids ON boat.id = ids.id
  WHERE boat.id IS NULL;

RETURN new_id;
END;
$$