Hide an error from the application using a servererror trigger?

Question

We have an application designed for an old oracle version which issues some sql which is no more supported in todays database version.
We want to use the application unchanged with a new database server.
Old Server Version: 7.3.4 (still in production...)
New Server Version: 10.2 or 11.2
The application issues an
ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS ;
which results in ORA-01986 and the application dies.
We would like to hide the error 01986 from the application using a trigger:
create or replace
trigger catch01986
  after servererror
  on schema
  begin
    if (ora_is_servererror (1986)) then
      null; -- what to do here? we want clear the ora-01986 from the error stack
    end if;
  end catch01986;How to handle the error, so that the alter session set ... statement is just ignored and no error code is returned to the application?
I asked already some days ago in Database-General Forum, but triggers belong to PL/SQL, so i repost here.
Tnx for help in advance!

Answer 1

Hi,
hoek wrote:
A totally weird and untested (and unable to test today) thought:
http://technology.amis.nl/blog/447/how-to-drive-your-colleagues-nuts-dbms_advanced_rewrite-oracle-10g
Very interesting for real dirty solution.
Does not work for my problem, DBMS_ADVANCED_REWRITE works only for select statements.
BEGIN
   SYS.DBMS_ADVANCED_REWRITE.DECLARE_REWRITE_EQUIVALENCE (
   'alter_session_equivalence',
   'ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS',
   'ALTER SESSION SET OPTIMIZER_MODE = RULE',
   FALSE);
END;
ORA-30389: the source statement is not compatible with the destination statement
ORA-00903: invalid table name
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 29
ORA-06512: at "SYS.DBMS_ADVANCED_REWRITE", line 185
ORA-06512: at line 2
30389. 00000 -  "the source statement is not compatible with the destination statement"
*Cause:    The SELECT clause of the source statement is not compatible with
           the SELECT clause of the destination statement
*Action:   Verify both SELECT clauses are compatible with each other such as
           numbers of SELECT list items are the same and the datatype for
           each SELECT list item is compatible
hoek wrote:You already had some trigger code, catching the error and sending it to null, why didn't that work?The trigger is fired when the error occurs, but after completion of the trigger, the error code is still delivered to the client.
I dont know how to handle the error within the trigger.
Does the client read the error stack and does it die after reading an error from the stack?The client just checks the error code. On error it terminates.
With the SERVERERROR TRIGGER i did the following tests:
Test 1: trigger does nothing
CREATE OR REPLACE
TRIGGER CATCH01986
  AFTER SERVERERROR
  ON SCHEMA
  BEGIN
    IF (ORA_IS_SERVERERROR (1986)) THEN
      NULL;
    END IF;
  END CATCH01986;
ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS;
ORA-01986: OPTIMIZER_GOAL is obsolete
01986. 00000 -  "OPTIMIZER_GOAL is obsolete"
*Cause:    An obsolete parameter, OPTIMIZER_GOAL, was referenced.
*Action:   Use the OPTIMIZER_MODE parameter.
-- Client Application reports errorcode 1986Test 2: Trigger raises NO_DATA_FOUND
CREATE OR REPLACE
TRIGGER CATCH01986
  AFTER SERVERERROR
  ON SCHEMA
  BEGIN
    IF (ORA_IS_SERVERERROR (1986)) THEN
      RAISE NO_DATA_FOUND;
    END IF;
  END CATCH01986;
ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS;
ORA-04088: error during execution of trigger 'AH.CATCH01986'
ORA-01403: no data found
ORA-06512: at line 9
ORA-01986: OPTIMIZER_GOAL is obsolete
04088. 00000 -  "error during execution of trigger '%s.%s'"
*Cause:    A runtime error occurred during execution of a trigger.
*Action:   Check the triggers which were involved in the operation.
-- Client Application reports errorcode 4088Test 3: Trigger raising an APPLICATION ERROR
CREATE OR REPLACE
TRIGGER CATCH01986
  AFTER SERVERERROR
  ON SCHEMA
  BEGIN
    IF (ORA_IS_SERVERERROR (1986)) THEN
        DBMS_STANDARD.RAISE_APPLICATION_ERROR(-20999, 'this makes no sense', true);
    END IF;
  END CATCH01986;
ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS;
ORA-00604: error occurred at recursive SQL level 1
ORA-20999: this makes no sense
ORA-06512: at line 10
ORA-01986: OPTIMIZER_GOAL is obsolete
00604. 00000 -  "error occurred at recursive SQL level %s"
*Cause:    An error occurred while processing a recursive SQL statement
           (a statement applying to internal dictionary tables).
*Action:   If the situation described in the next error on the stack
           can be corrected, do so; otherwise contact Oracle Support.
-- Client Application reports errorcode 604Test 4: Adding an EXCEPTION part to the trigger does not help, this will catch only exceptions raised while the trigger executes:
CREATE OR REPLACE
TRIGGER CATCH01986
  AFTER SERVERERROR
  ON SCHEMA
  BEGIN
    IF (ORA_IS_SERVERERROR (1986)) THEN
        DBMS_STANDARD.RAISE_APPLICATION_ERROR(-20999, 'this makes no sense', true);
    END IF;
  EXCEPTION
    WHEN OTHERS THEN
      NULL;
  END CATCH01986;
ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS;
ORA-01986: OPTIMIZER_GOAL is obsolete
01986. 00000 -  "OPTIMIZER_GOAL is obsolete"
*Cause:    An obsolete parameter, OPTIMIZER_GOAL, was referenced.
*Action:   Use the OPTIMIZER_MODE parameter.
-- Client Application reports errorcode 1986So i do not know what to do inside the trigger to clean the error stack so that the client will receive no errorcode.