Adaptive Server maintains the total density for each prefix subset of columns in a composite index. If two tables are being joined on multiple leading columns of a composite index, the optimizer uses the appropriate density for an index when estimating the cost of a join using that index. In a 10,000-row table with an index on seven columns, the entire seven-column key might have a density of 1/10,000, while the first column might have a density of only 1/2, indicating that it would return 5000 rows.
