ORA-12899, value too large

How can I avoid ORA-12899, value too large for column error? I have a database and program which are not made by me. I have sources, so I could make changes, but I will not if it can be avoided. The program reads a file and stores it to the database. Reading a file can take days. Some of the values are quite large (last night the program stopped because the value contained over 8000 characters) but I guess the large values are such that they could be truncated.

Since I don't like to change the program (I changed the field from varchar2(1500) to varchar2(4000)), I like to make the truncation automatic. It occurred to me, that I could use triggers. INSTEAD OF trigger seemed fine (instead of inserting the value directly, I'd insert SUBSTR(value,1,4000)), but Oracle told that INSTEAD OF trigger can be used only with views.

Then I tried BEFORE trigger, but it did nothing:

create of replace trigger testing_trigger before insert or update of value on testing for each row
begin

   :new.value := substr(:new.value,1,10); end;
/

(I used a test table TESTING, with fields ID and VALUE; value was varchar2(10));

But,

INSERT INTO TESTING VALUES(2,'01234567890'); gave ORA-12899

so it seems that the trigger did nothing.

So question is: can I use triggers in this case? If not, is there any way to tell Oracle to truncate itself? (OS: Linux, Oracle: 10g)

--
Arto Viitanen
CSC, Espoo, Finland