Integrating information by outerjoins and full disjunctions

Abstract

Our motivation is the piecing together of tidbits of information found on the 'web' into a usable information structure. The problem is related to that of computing the natural outerjoin of many relations in a way that preserves all possible connections among facts. Such a computation has been termed a 'full disjunction' by Galindo-Legaria. We are thus led to ask the question of when a full disjunction can be computed by some sequence of natural outerjoins. The answer involves a concept of from Fagin [1983] called 'γ-acyclic hypergraphs.' We prove that there is a natural outerjoin sequence producing the full disjunction if and only if the set of relation schemes forms a connected, γ-acyclic hypergraph.