CREATE OR REPLACE FUNCTION fn_category_get_childs_v2(id_pai integer)
RETURNS integer[] AS
$BODY$

DECLARE

ids_filhos integer array;


BEGIN

SELECT array (

SELECT category_id FROM category WHERE category_id IN (
(WITH RECURSIVE parent AS
(
    SELECT category_id , parent_id  from category WHERE category_id = id_pai
    UNION ALL
    SELECT t.category_id , t.parent_id FROM parent
    INNER JOIN category t ON parent.category_id =  t.parent_id
)

SELECT category_id FROM  parent
WHERE category_id <> id_pai
) )


 ) into ids_filhos;

return ids_filhos;

END;

select *
from teste1_elements
where category_id in (select * from fn_category_get_childs_v2(12))

select *
from teste1_elements
where category_id=any(select * from fn_category_get_childs_v2(12)))

ERROR:  operator does not exist: integer = integer[]
LINE 1: select * from teste1_elements where category_id in (select *...
                                                    ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

SELECT * from fn_category_get_childs_v2(12)

'{30,32,34,20,19,18,17,16,15,14}'

select *
from teste1_elements
where category_id = any(fn_category_get_childs_v2(12))

WITH RECURSIVE children AS (
    SELECT category_id
    FROM   category
    WHERE  parent_id = id_pai

    UNION ALL
    SELECT c.category_id
    FROM   children ch
    JOIN   category c ON c.parent_id = ch.category_id
    )
SELECT *
FROM   children;

CREATE OR REPLACE FUNCTION f_cat_children(_id_pai int, OUT category_id int)
  RETURNS SETOF int LANGUAGE SQL AS
$func$
   WITH RECURSIVE children AS (
      SELECT c.category_id
      FROM   category c
      WHERE  c.parent_id = $1

      UNION ALL
      SELECT c.category_id
      FROM   children ch
      JOIN   category c ON c.parent_id = ch.category_id
      )
   SELECT *
   FROM   children;
$func$;

SELECT t.*
FROM   teste1_elements t
JOIN   f_cat_children(12) USING (category_id)

WITH RECURSIVE children AS (
    SELECT category_id
    FROM   category
    WHERE  parent_id = id_pai

    UNION ALL
    SELECT c.category_id
    FROM   children ch
    JOIN   category c ON c.parent_id = ch.category_id
    )
SELECT t.*
FROM   children
JOIN   teste1_elements t USING (category_id);