teste=# select * from tabela_codigo;
 codigo
--------
      1
      2
      3
      5
      6
      8
     10
(7 rows)

teste=# select a.codigo from gera_serie(1, (select max(codigo) from tabela_codigo), 1) as a(codigo)
teste-# except
teste-# select codigo from tabela_codigo;
 codigo
--------
      4
      7
      9
(3 rows)

teste=# select a.codigo from gera_serie(1, (select max(codigo) from tabela_codigo), 1) as a(codigo)
teste-# except
teste-# select codigo from tabela_codigo
teste-# order by codigo
teste-# limit 1
teste-# ;
 codigo
--------
      4
(1 row)

CREATE OR REPLACE FUNCTION gera_serie(int8, int8, int8)
  RETURNS SETOF int8 AS
$BODY$
declare
inicio alias for $1;
fim alias for $2;
passo alias for $3;
i bigint := inicio;
begin
while i <= fim loop
  return next i;
  i := i + passo;
end loop;
return;
end;
$BODY$
  LANGUAGE 'plpgsql' STABLE;

CREATE OR REPLACE FUNCTION GERA_SERIE(INICIO NUMBER,FIM NUMBER, PASSO NUMBER) 
  RETURN NUMBER IS CODIGO NUMBER(7) :=INICIO;   
BEGIN

WHILE CODIGO <= FIM LOOP 
  RETURN(CODIGO);  ---Talvez falte algo com NEXT ??? 
  CODIGO := CODIGO + PASSO; 
END LOOP; 
END GERA_SERIE;

SELECT * FROM GERA_SERIE(1,(SELECT MAX(CD_APLICATIVO) FROM APLICATIVOS),1)
EXCEPT 
SELECT CD_APLICATIVO FROM APLICATIVOS;

SELECT *  FROM  GERA_SERIE(1,30,1);

create type codigo as table of number

CREATE OR REPLACE FUNCTION GERA_SERIE(INICIO NUMBER,FIM NUMBER, PASSO NUMBER)
  RETURN codigo PIPELINED IS CODIGO NUMBER(7) :=INICIO;   
BEGIN

WHILE CODIGO <= FIM LOOP
  CODIGO := CODIGO + PASSO;
  PIPE ROW(codigo);
END LOOP;
RETURN;
END GERA_SERIE;

create type codigo as table of number;