Bug? using MAX() function on char(1) column returns something larger

Question

details:
-- we have a complex nested query, where we are essentially returning the max() value into a variable
-- the max() function is being used on a char(1) column
-- where MAX() is part of an inner select, we have started getting
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
errors.
SELECT MAX(X) INTO var FROM ... works in 9i and 10g
SELECT X INTO var FROM (SELECT MAX(X) X FROM ... worked in 9i, does not work in 10g!
-- We never had problems with the code until upgrading to 10g release 2.
-- the Solution is to cast the final value with TO_CHAR(). The cast MUST be done at the outer most point of the select:
SELECT TO_CHAR(X) INTO var FROM (SELECT MAX(X) X FROM ... works
SELECT X INTO var FROM (SELECT TO_CHAR(MAX(X)) X FROM ... causes an error!
The following script demonstrates the issue, and includes the solution:
* October 3, 2006
* Possible SQL bug introduced with Oracle 10G
* Natalie Gray DBA/Developer, Environment Canada
* Description:
* Have discovered a problem with using the MAX() function
* on columns of type char(1)
* only an issue when used in an inner select
* solution (see test 4)
CREATE TABLE SQL_BUG_TEST
X NUMBER,
Y NUMBER,
Z CHAR(1)
INSERT INTO SQL_BUG_TEST (X, Y, Z)
VALUES (1,1,'A');
INSERT INTO SQL_BUG_TEST (X, Y, Z)
VALUES (1,1,'B');
INSERT INTO SQL_BUG_TEST (X, Y, Z)
VALUES (1,2,'C');
INSERT INTO SQL_BUG_TEST (X, Y, Z)
VALUES (1,2,'D');
DECLARE
TYPE REC IS RECORD (
      x SQL_BUG_TEST.X%TYPE,
      y SQL_BUG_TEST.Y%TYPE,
      z SQL_BUG_TEST.Z%TYPE
v_rec REC;
BEGIN
      -- DISPLAY THE TABLE DATA
      BEGIN
             DBMS_OUTPUT.PUT_LINE('TABLE DATA:');
            DBMS_OUTPUT.PUT_LINE('');
             DBMS_OUTPUT.PUT_LINE('SELECT * FROM SQL_BUG_TEST ORDER BY X,Y,Z;');
             FOR crs IN (SELECT *
                                 FROM SQL_BUG_TEST
                           ORDER BY X,Y,Z) LOOP
                DBMS_OUTPUT.PUT_LINE(':'||crs.X||':'||crs.Y||':'||crs.Z);
            END LOOP;
      EXCEPTION WHEN OTHERS THEN
             DBMS_OUTPUT.PUT_LINE(SQLERRM);       
      END;
      -- TEST 1
      -- returning result from MAX into a variable when the MAX is in the outer most select
      -- does not cause an error
      BEGIN
             DBMS_OUTPUT.PUT_LINE('*****************************************************');
             DBMS_OUTPUT.PUT_LINE('TEST 1');
            DBMS_OUTPUT.PUT_LINE('');
             DBMS_OUTPUT.PUT_LINE('SELECT X, Y, MAX(Z) Z');
            DBMS_OUTPUT.PUT_LINE('FROM SQL_BUG_TEST');
            DBMS_OUTPUT.PUT_LINE('GROUP BY X,Y');
            FOR crs IN (SELECT X, Y, MAX(Z) Z
                                 FROM SQL_BUG_TEST
                           GROUP BY X, Y
                           ORDER BY X,Y,Z) LOOP
                 DBMS_OUTPUT.PUT_LINE(':'||crs.X||':'||crs.Y||':'||crs.Z);
            END LOOP;
      EXCEPTION WHEN OTHERS THEN
           DBMS_OUTPUT.PUT_LINE(SQLERRM);
      END;
      -- TEST 2
      -- returning MAX() from an inner select to an outer select and then into a variable
      -- causes an error
      BEGIN
             DBMS_OUTPUT.PUT_LINE('*****************************************************');
             DBMS_OUTPUT.PUT_LINE('TEST 2');
            DBMS_OUTPUT.PUT_LINE('THIS DID NOT CAUSE AN ERROR WITH ORACLE 9i');
            DBMS_OUTPUT.PUT_LINE('');
             DBMS_OUTPUT.PUT_LINE('SELECT * INTO v_rec');
            DBMS_OUTPUT.PUT_LINE('FROM');
            DBMS_OUTPUT.PUT_LINE('(SELECT X, Y, MAX(Z) Z');
            DBMS_OUTPUT.PUT_LINE('FROM SQL_BUG_TEST');
            DBMS_OUTPUT.PUT_LINE('GROUP BY X,Y)');
            DBMS_OUTPUT.PUT_LINE('WHERE Y = 1');
            SELECT * INTO v_rec
            FROM
            (SELECT X, Y, MAX(Z) Z
             FROM SQL_BUG_TEST
             GROUP BY X, Y)
            WHERE Y = 1;
            DBMS_OUTPUT.PUT_LINE(':'||v_rec.X||':'||v_rec.Y||':'||v_rec.Z);
      EXCEPTION WHEN OTHERS THEN
           DBMS_OUTPUT.PUT_LINE(SQLERRM);
      END;
      -- TEST 3
      -- casting the result from MAX to char before returning to the outer select
      -- still causes an error
      BEGIN
             DBMS_OUTPUT.PUT_LINE('*****************************************************');
             DBMS_OUTPUT.PUT_LINE('TEST 3');
            DBMS_OUTPUT.PUT_LINE('');
             DBMS_OUTPUT.PUT_LINE('SELECT * INTO v_rec');
            DBMS_OUTPUT.PUT_LINE('FROM');
            DBMS_OUTPUT.PUT_LINE('(SELECT X, Y, to_char(MAX(Z)) Z');
            DBMS_OUTPUT.PUT_LINE('FROM SQL_BUG_TEST');
            DBMS_OUTPUT.PUT_LINE('GROUP BY X,Y)');
            DBMS_OUTPUT.PUT_LINE('WHERE Y = 1');
            SELECT * INTO v_rec
            FROM
            (SELECT X, Y, to_char(MAX(Z)) Z
             FROM SQL_BUG_TEST
             GROUP BY X, Y)
            WHERE Y = 1;
            DBMS_OUTPUT.PUT_LINE(':'||v_rec.X||':'||v_rec.Y||':'||v_rec.Z);
      EXCEPTION WHEN OTHERS THEN
           DBMS_OUTPUT.PUT_LINE(SQLERRM);
      END;
      -- TEST 4 - SOLUTION
      -- the return value of MAX must be cast with to_char at the point where it is assigned to
      -- variable (outer most select)
      BEGIN
             DBMS_OUTPUT.PUT_LINE('*****************************************************');
             DBMS_OUTPUT.PUT_LINE('TEST 4 SOLUTION');
            DBMS_OUTPUT.PUT_LINE('');
             DBMS_OUTPUT.PUT_LINE('SELECT X, Y, TO_CHAR(Z) Z INTO v_rec');
            DBMS_OUTPUT.PUT_LINE('FROM');
            DBMS_OUTPUT.PUT_LINE('(SELECT X, Y, MAX(Z) Z');
            DBMS_OUTPUT.PUT_LINE('FROM SQL_BUG_TEST');
            DBMS_OUTPUT.PUT_LINE('GROUP BY X,Y)');
            DBMS_OUTPUT.PUT_LINE('WHERE Y = 1');
            SELECT X, Y, TO_CHAR(Z) Z INTO v_rec
            FROM
            (SELECT X, Y, MAX(Z) Z
             FROM SQL_BUG_TEST
             GROUP BY X, Y)
            WHERE Y = 1;
            DBMS_OUTPUT.PUT_LINE(':'||v_rec.X||':'||v_rec.Y||':'||v_rec.Z);
      EXCEPTION WHEN OTHERS THEN
           DBMS_OUTPUT.PUT_LINE(SQLERRM);
      END;
END;

Answer 1

I certainly looks like a bug, but you should raise an iTAR on Metalink since Oracle does not monitor this forum.
I was able to replicate your results on my 10.2.0.1 database.
There is an easier workaround than yours. Try
ALTER TABLE sql_bug_test MODIFY (z VARCHAR2(1));That seems to eliminate the problem on my instance.
John