SQL Update from Select problem

This SQL runs indefinitely and takes 95% CPU. However, if I create a temporary table (say R_VEHICLE_TEMP) and modify the SQL slightly to update that instead of R_VEHICLE, the SQL only takes 5 seconds to run correctly. Why can't I select and update from/to the same table?

UPDATE R_VEHICLE rv1
SET (RANK, VPCT) = ( select rank,vpct
 from (SELECT w_vhc_id,
  DENSE_RANK() OVER(ORDER BY VPSUM DESC) AS RANK,   PERCENT_RANK() OVER(ORDER BY VPSUM ASC) AS VPCT   FROM R_VEHICLE rv2
  WHERE VERSION_ID = 19
  and vpsum > 0) v1
 where rv1.w_vhc_id = v1.w_vhc_id)
where VERSION_ID = 19
and vpsum > 0
/

Thanks for any help,
Steve Received on Thu Dec 19 2002 - 07:44:56 CET