Cost-Based.Oracle.Fundamentals

The typical cost of using a B-tree index comprises three components—the depth of the index, based on the blevel; the number of index leaf blocks that will be visited, based on leaf_blocks; and the number of visits to table blocks, based on the clustering_factor. The clustering_factor of an index is a primary indicator of how desirable the index appears to be to the optimizer. The drawback is that you need to compare the clustering_factor to the number of rows and blocks in the table—the number by itself is effectively meaningless.