Dataytpe mismatch error in GROUP BY Clause

Question

I have a function(undergoing testing) which returns column values separated by comma. Since this function returns a huge number of values (track_id) which a varchar2 type can't store i've made the return type to be CLOB.
CREATE OR REPLACE FUNCTION get_shp_no_list
p_shp_nbr IN VARCHAR2
) RETURN clob
AS
  type shp_list_type is table of shp_trkg_dtl.track_ID%type;
  v_shp shp_list_type;
  v_ship_list  CLOB; 
BEGIN
     -- get all track_ids for the selected shp_nbr
     SELECT DISTINCT (track_id )
     BULK COLLECT INTO v_shp
     FROM shp_trkg_dtl
     WHERE shp_id = p_shp_nbr
     ORDER BY track_ID;
     FOR i IN v_shp.FIRST .. v_shp.LAST LOOP
         IF v_ship_list IS NULL THEN
            v_ship_list := v_shp(i);
         ELSE
            v_ship_list := v_ship_list || ',' || v_shp(i);
         END IF;
     END LOOP;     
     --return the track_id list
     RETURN v_ship_list;
EXCEPTION
  WHEN OTHERS THEN
    RETURN NULL;
END get_shp_no_list;
/This function works fine when called as standalone. But when i use this function in a SELECT in a stored proc i am getting error. My query looks like
SELECT .......
FROM
(SELECT sd.shp_id,get_shp_no_list(sd.shp_id) trackids,
FROM shp_trkg_dtl sd
WHERE sd.shp_id IN  ('LPNR0456T','LPNR0498Y','LPNG0471G','LPNG783H') source1
INNER JOIN ........................
Group By source1.trackids  -- -<font color="red"><b> line where error has occured</b></font>
ERROR at line 15:
ORA-00932: inconsistent datatypes: expected - got CLOBAny thoughts?

Answer 1

Taken from the 10.2 manual:
Restrictions on the GROUP BY Clause:
This clause is subject to the following restrictions:
You cannot specify LOB columns, nested tables, or varrays as part of expr.
The expressions can be of any form except scalar subquery expressions.
If the group_by_clause references any object type columns, then the query will not be parallelized.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_10002.htm#i2065646