Error raised when concatenating to CLOB using implicit to_char conversion

Question

Hi there,
Please consider the code below (Oracle 11.1.0.7.0):
DECLARE
  c CLOB;
BEGIN
  FOR i IN 1..10
  LOOP
    -- Please uncomment ONE of the lines below
    c := c ||  LPAD('x', 8000, 'z') || i; -- raises error ora-06502
    --c := c || (LPAD('x', 8000, 'z') || i);  -- works
    --c := c ||  LPAD('x', 8000, 'z') || to_char(i);  -- works
  END LOOP;
END;Ok, maybe making use of implicit type conversion is not the best thing to do, but just out of curiosity is there any explanation for this error to happen?
It seems that, when implicit conversion is involved without the aid of brackets, Oracle tries to convert the whole thing to varchar2 before concatenating it to the clob, raising the error.
Thanks
Luis

Answer 1

Hi,
Accordingly link below, I think this behavior can be explained
http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28370/datatypes.htm#CIHGBADF
Case 1
c := c ||  LPAD('x', 8000, 'z') || i
How it going
1. Evaluate expression  res1 = LPAD('x', 8000, 'z')
2. Next oracle must to evaluate  c||res1 where c is clob and res1 is varchar2
3. Oracle can and choose implicit conversation res1 to clob
4. Evaluate expression  res2 = c||to_clob(res1);  where res2 now is clob
5. Next oracle must to evaluate  res2||i where res2 is clob and i is number
6. Oracle can implicitly convert number only to varchar2 not to clob so he choose implicit conversation i to varchar2
6.1 Oracle implicitly try convert res2 to varchar2 and when res2 size more then 32627 then he raise error ORA-06502Case 2
c := c || (LPAD('x', 8000, 'z') || i)
How it going
1. Evaluate expression  res1 = LPAD('x', 8000, 'z')
2. Next oracle must to evaluate  res1||i where res1 is varchar2 and i is number
3. So he convert i to varchar2 and evaluate res2 = res1||to_char(i)
4. Next oracle must to evaluate  c||res2 where res2 is varchar2 and c is clob
5. So he convert res2 to clob and evaluate c||to_clob(res2)Case 3
c := c ||  LPAD('x', 8000, 'z') || to_char(i)
How it going
0. Evaluate res0 = to_char(i)
1. Evaluate expression  res1 = LPAD('x', 8000, 'z')
2. Next oracle must to evaluate  c||res1 where c is clob and res1 is varchar2
3. Oracle can and choose implicit conversation res1 to clob
4. Evaluate expression  res2 = c||to_clob(res1);  where res2 now is clob
5. Next oracle must to evaluate  res2||res0 where res2 is clob and res0 is varchar2
6. He evaluate  to_clob(res0)||res2