-- почему-то FETCH FIRST нельзя использовать во внешнем селекте
CREATE FUNCTION xxx (n INTEGER)
     RETURNS TABLE (m INTEGER)
     LANGUAGE SQL
     READS SQL DATA
     NO EXTERNAL ACTION
     RETURN
       SELECT person_Id
         FROM APLUS.SEGMENT
         ORDER BY person_id
         FETCH FIRST  10  ROWS ONLY
    ;
-- в FETCH FIRST нельзя использовать переменную вместо константы
CREATE FUNCTION xxx (n INTEGER)
     RETURNS TABLE (m INTEGER)
     LANGUAGE SQL
     READS SQL DATA
     NO EXTERNAL ACTION
     RETURN
       WITH temp(person_id) AS (
       SELECT person_Id
         FROM APLUS.SEGMENT
         ORDER BY person_id
         FETCH FIRST n ROWS ONLY
     ) select * from temp;

-- FETCH FIRST во внутреннем селекте, константа
CREATE FUNCTION xxx (n INTEGER)
     RETURNS TABLE (m INTEGER)
     LANGUAGE SQL
     READS SQL DATA
     NO EXTERNAL ACTION
     RETURN
       WITH temp(person_id) AS (
       SELECT person_Id
         FROM APLUS.SEGMENT
         ORDER BY person_id
         FETCH FIRST  10  ROWS ONLY
     ) select * from temp;

CREATE FUNCTION xxx (n INTEGER)
     RETURNS TABLE (m INTEGER)
     LANGUAGE SQL
     READS SQL DATA
     NO EXTERNAL ACTION
     RETURN
  SELECT person_id FROM
    (SELECT
       ROW_NUMBER() OVER() AS r#, person_id
     FROM APLUS.SEGMENT
    ) AS e
  WHERE r# <= n

  DECLARE rcount IINTEGER;
  UPDATE ... ;
  GET DIAGNOSTICS rcount = ROW_COUNT; -- это специальная синтаксическая конструкция

  SET rcount =  0 ;
  FOR xxx AS 
    SELECT ..
  DO
    INSERT INTO T1 VALUES (...);
    INSERT INTO T2 VALUES(IDENTITY_VAL_LOCAL()...);
    SET rcount=rcount+ 1 ;
  END FOR;