Simplification of outer joins

Authors:
Gautam Bhargava;Piyush Goel;Balakrishna R. Iyer
Affiliations:
Silicon Graphics Computer Systems, Mountain View, California;University of Michigan, Ann Arbor, Michigan;IBM Santa Teresa Laboratory, San Jose, California
Venue:
CASCON '95 Proceedings of the 1995 conference of the Centre for Advanced Studies on Collaborative research
Year:
1995

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Abstract

The removal of redundant outer joins is essential for the reassociation of outer joins with other binary operations. In this paper, we present a set of comprehensive algorithms that employ the properties of strong predicates along with the properties of SQL's projection, intersection, union and except operations in order to remove redundant outer joins from a complex query. For the purpose of query simplification, we generate additional projections by determining the keys. Our algorithm for generating keys is based on a novel concept of weak bindings that is essential for the queries containing outer joins. Our algorithm for converting outer joins to joins is based on a novel concept of join-reducibility, and computes a fixed point solution for a general query. If the query does not contain intersect and outer join operations, then the fixed point solution is not required.