Reading bytes from CLOB not BLOB

Question

I know that DBMS_LOB.SUBSTR can read number of bytes (for BLOBs) or characters (for CLOBs).
What I want is to read bytes from BLOB instead of characters.
The following tries to read 4000 bytes. Unfortunately It fails to read 4000 bytes if size of CLOB_COLUMN is bigger than 4000 bytes.
select dbms_lob.substr(utl_raw.cast_to_raw(CLOB_COLUMN),4000,1) from dualIs it possible to get only these 4000 bytes from CLOB_COLUMN?
Thank you in advance.

Answer 1

Welcome to the forum.
Unfortunately It fails to read 4000 bytes if size of CLOB_COLUMN is bigger than 4000 bytes.Could it be you're mixing up things?
DBMS_LOB.SUBSTR reads bytes from BLOBS, characters from CLOBS.
"amount Number of bytes (for BLOBs) or characters (for CLOBs) to be read."
http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14258/d_lob.htm#sthref3199
No need to cast_to_raw, afaik:
SQL> create table t as select to_clob(lpad('*', 10000, '*')) clob_col from dual;
Table created.
SQL> desc t;
Name                                                                   Null?    Type
CLOB_COL                                                                        CLOB
SQL> select substr(clob_col,1, 4000) substr
  2  ,      length(substr(clob_col,1, 4000)) substr_lngth
  3  ,      dbms_lob.substr(clob_col, 4000, 1) lob_substr
  4  ,      length(dbms_lob.substr(clob_col, 4000, 1)) lob_substr_length
  5  from t;
SUBSTR                                                                           SUBSTR_LNGTH
LOB_SUBSTR
LOB_SUBSTR_LENGTH
********************************************************************************         4000
             4000
1 row selected.