For each prefix subset, two density values are stored:
Range cell density, used for search arguments
Total density, used for joins
Range cell density represents the average number of duplicates of all values that are represented by range cells in the histogram. Total density represents the average number of duplicates for all values, those in both frequency and range cells. Total density is used to estimate the number of matching rows for joins and for search arguments whose value is not known when the query is optimized.
