Workaround for ORA-22835: Buffer too small for CLOB to CHAR or BLOB to RAW conversion

ORA-22835 was encountered following a database upgrade. This post explains why it happened and provides an undocumented workaround.

Demonstration

Oracle versions 9i and 10.1 have a bug in a LOB to CHAR conversion. When the value is larger than 4000 bytes, it is silently truncated without dropping an error (see: Ora-22835 - CLOB Larger Than 4000 Inserted Into Varchar2(4000) Column Is Silently Truncated (Doc ID 388512.1)). This can become an issue following a database upgrade since newer versions start dropping an ORA-22835 error. This happened in this topic on SQL.RU. Here is a demonstration:

SQL> !oerr ora 22836
22836, 00000, "Event to turn on lob to char/raw silent truncation"
// *Document: NO
// *Cause:    N/A
// *Action:   Do not throw error 22835 for truncation during LOB to CHAR/RAW
//            conversion. Truncate the data instead.

SQL>
SQL> create table t(vc varchar2(4000));

Table created.

SQL>
SQL> var c clob
SQL>
SQL> begin
  2    select xmltype.createxml(cursor(select * from all_objects)).getClobVal()
  3      into :c
  4      from dual;
  5  end;
  6  /

PL/SQL procedure successfully completed.

SQL>
SQL> exec dbms_output.put_line(dbms_lob.getlength(:c))
12273513

PL/SQL procedure successfully completed.

SQL>
SQL> insert into t values (to_char(:c));
insert into t values (to_char(:c))
                      *
ERROR at line 1:
ORA-22835: Buffer too small for CLOB to CHAR or BLOB to RAW conversion (actual:
12273513, maximum: 4000)


SQL>
SQL> alter session set events '22836 level 1';

Session altered.

SQL>
SQL> insert into t values (to_char(:c));

1 row created.

SQL>
SQL> select length(vc) from t;

LENGTH(VC)
----------
      4000

SQL>
SQL> select banner_full from v$version;

BANNER_FULL
--------------------------------------------------------------------------------
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.9.0.0.0

Conclusion

Event 22836 can be used to suppress the ORA-22835 error and enable the old behavior - LOB values larger than 4000 bytes will be silently truncated without throwing an error. This is not something that can be considered as a long term solution, however, it can become handy in some cases when changing the code is not practical or not feasible.