The default of 0 means that the optimizer chooses the best serial plan. A value of 100 means that the optimizer uses each object's degree of parallelism in computing the cost of a full table scan operation.
Low values favor indexes, and high values favor table scans.
Cost-based optimization is always used for queries that reference an object with a nonzero degree of parallelism. For such queries, a RULE hint or optimizer mode or goal will be ignored. All Rights Reserved.