Error When Query Gets Blocked

Question

I am wondering if there is a way that I can make a query error if it is going to (or does) get blocked during an insert where not exists. I have multiple sessions occasionally trying to insert the same row into a table. When the second session tries to insert, I want it to simply error instead of block until the first session commits. Any idea on how to accomplish this?
My first attempt was to select for update nowait, but that doesn’t work for rows not insert committed into the table. I feel like I should be able to do this with transaction isolation, but that does not seem to work either. I have some SQL below trying to use transaction isolation to accomplish this.
Any help would be greatly appreciated. Thanks.
CREATE TABLE t$test
    dummy NUMBER,
    CONSTRAINT t$test_pk
    PRIMARY KEY (dummy)
INSERT INTO t$test
     VALUES (1);
-- Session 2
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
INSERT INTO t$test
   SELECT 1
     FROM DUAL
    WHERE NOT EXISTS (SELECT 1
                        FROM t$test
                       WHERE dummy = 1);
-- I want this to error, but instead it still blocks

Answer 1

That was the approach I was taking. Problem is, with 25 batches of this running, the amount of blocking was putting a tremendous load on the database.
After some research, I could have implemented my own row locking scheme using the DBMS_LOCK package, but given the amount of data I have, I would need to create way too many locks than what is feasible.
I was hoping that Oracle would have a method for non-blocking write (error instead). Unless I can find one, I am just going to sacrifice some of my batch capabilities and commit more frequently to minimize the number of locks at one time.
I'd still be interested to hear other solutions if they're out there. Thanks for the help Tubby.
Below is the PL/SQL I was considering for my row locking scheme:
DECLARE
   v_lockname   VARCHAR2 (255) := 'lock_' || :dummy;
   v_lockid     LONG;
   v_result     PLS_INTEGER;
BEGIN
   DBMS_LOCK.allocate_unique (v_lockname, v_lockid);
   v_result := DBMS_LOCK.request (v_lockid, 6, 0, TRUE);
   IF (v_result = 0)
   THEN
      INSERT INTO t$test
         SELECT :dummy
           FROM DUAL
          WHERE NOT EXISTS (SELECT 1
                              FROM t$test
                             WHERE dummy = :dummy);
   -- Other return value handling needed
   END IF;
END;