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2001 Technical Reports

Holes in Joins

Jarek Gryz and Dongming Liang

Technical Report CS-2001-03

York University

June 18, 2001

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 combinations as ranges of attributes that do not appear together. We sketch an algorithm for finding such combinations and present experimental results from two 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.

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