For hash-based access methods, the optimizer does not compute an optimal degree of parallelism; instead, it uses the number of worker processes specified by the max scan parallel degree parameter. It is up to the System Administrator to set max scan parallel degree to an optimal value for the Adaptive Server system as a whole. A general rule of thumb is to set this parameter to no more than 2 or 3, since it takes only 2–3 worker processes to fully utilize the I/O of a given physical device.
