Error while converting CLOB to varchar using DBMS_LOB.SUBSTR() in Oracle11g

Question

Hi
Whenever I am using DBMS_LOB.SUBSTR(columnname,4000,1) package for a clob column in a simple Select Query, the following error is thrown for Oracle 11g version.
ORA-06502: PL/SQL: numeric or value error: character string buffer too small ORA-06512: at line 1.
Please find the installation details of the database and the character set
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE     11.2.0.1.0     Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
PARAMETER VALUE
NLS_LANGUAGE AMERICAN
NLS_TERRITORY AMERICA
NLS_CURRENCY $
NLS_ISO_CURRENCY AMERICA
NLS_NUMERIC_CHARACTERS .,
NLS_CHARACTERSET AL32UTF8
NLS_CALENDAR GREGORIAN
NLS_DATE_FORMAT DD-MON-RR
NLS_DATE_LANGUAGE AMERICAN
NLS_SORT BINARY
NLS_TIME_FORMAT HH.MI.SSXFF AM
NLS_TIMESTAMP_FORMAT DD-MON-RR HH.MI.SSXFF AM
NLS_TIME_TZ_FORMAT HH.MI.SSXFF AM TZR
NLS_TIMESTAMP_TZ_FORMAT DD-MON-RR HH.MI.SSXFF AM TZR
NLS_DUAL_CURRENCY $
NLS_COMP BINARY
NLS_LENGTH_SEMANTICS BYTE
NLS_NCHAR_CONV_EXCP FALSE
NLS_NCHAR_CHARACTERSET AL16UTF16
NLS_RDBMS_VERSION 11.2.0.1.0
It is working fine if I reduce from 4000 to 3500 . But I want to use 4000.
Please let me know if any solution

Answer 1

you are using a multibyte character set:
NLS_CHARACTERSET AL32UTF8
so each character takes between 1 and 4 bytes of storage.
a varchar2(4000) column can only hold 4000 BYTES. Regardless of the character set - varchar2 is limited to 4000 bytes.
In a single byte character set, that is 4000 characters as a character = a byte
In your character set, a varchar2(4000) can hold somewhere between 1000 and 4000 characters - depending on what the characters are.
So, you must have some CLOB whose first 4000 characters include at least one "more than one byte" character. That won't fit into a varchar2(4000)
Your approach of backing off the substr size is reasonable (and you'll need to remember that in your application - end users can type in as little as 1000 characters and get an error about the column being too small!) as it reduces the number of BYTES to be belong 4000.