Causes for "Enq: TM - contention" on a table with conventional path insert?

Folks,

I have a situation like this:

10.2.0.2 EE on Sparc 64bit

An insert statement of the form ...

INSERT INTO target_table

            (...)
   SELECT /*+ ORDERED FULL (t1) FULL(t2) FULL(t3) */

          ...
     FROM t1, t2, t3, t4
    WHERE t1.col1 = t2.col2
      AND t1.col3 = t3.col4
      AND t1.col1 >= :b2 
      AND t1.col1 < :b1

The target table is range partitioned with 8 ranges, and 8 copies of the above query run simultaneously to load the table using different :b1 and :B2 values. Each statement loads a single partition but the optimizer has no way of deducing that fact. (Not my design!)

There are no indexes on the table at the time of load.

There are no constraints on the table at all.

Oracle is serializing the inserts so that only one session can insert into the target table at a time. I would expect this if I was performing direct path load, maybe if there were bitmap indexes, or if there were FK's involved perhaps, but I've ruled out all of those.

V$SESSION_WAIT shows the following:

SID: 394
SEQ#: 79
Event: enq: TM - contention
P1TEXT: name|mode
...
P1RAW: 00000000544D0006 -- ie. a TM Exclusive?

.. and the P2 parameter gives the object_id for the target table in the insert statement

So I'm a bit at a loss to know what else could cause this, or what tests to run next time i execute this process in order to get more data to help analyze it.

Thoughts?

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