The query optimizer processes join predicates the same way it processes search arguments, in that it uses statistics, number of rows in the table, index heights, and the cluster ratios for the index and data pages to determine which index and join method provides the cheapest access. In addition, the query optimizer also uses join density estimates derived from join histograms that give accurate estimates of qualifying joining rows and the rows to be scanned in the outer and inner tables. The query optimizer also must decide on the optimal join ordering that will yield the most efficient query plan. The next sections describe the key techniques used in processing joins.
