Query optimization via empty joins

Abstract

A join of two relations in real databases is usually much smaller than their Cartesian product. This means that most of the combinations of tuples in the crossproduct of the respective relations do not appear together in the join result. We characterize these missing combinations as ranges of attributes that do not appear together and present experimental results on their discovery from real data sets. We then explore potential applications of this knowledge to query optimization. By modeling empty joins as materialized views, we show how knowledge of these regions can be used to improve query performance.