A More Aggressive Use Of Views To Extract Information

Abstract

Much recent work has focussed on using views to evaluate queries. More specifically, queries are rewritten to refer to views instead of the base relations over which the queries were originally written. The motivation is that the views represent the only ways in which some information source may be accessed. Another use of views that has been overlooked becomes important especially when no alent rewriting of a query in terms of views is possible: even though we cannot use the views to get all the answers to the query, we can still use them to deduce as many answers as possible. In many global information applications, the notion of equivalence used is often too restrictive. We propose a notion of pseudo-equivalence that allows more queries to be rewritten usefully: we show that if a query has an equivalent rewriting, the query also has a pseudo-equivalent rewriting. The converse is not true in general. In particular, when the views are conjunctive, we show that all Datalog queries over the source do have a pseudo-equivalent Datalog query over the views. We reduce the problem of finding pseudo-equivalent queries to that of rewriting Horn queries with Skolem functions as Datalog queries. We present an algorithm for the class of term-bounded Horn queries. We discuss extending the problem to larger classes of Horn queries, other non-Horn queries that result from "inverting" Datalog views and adding functional dependencies. The theory and methods developed in our work have important uses in query mediation between heterogeneous sources, automatic join discovery and view updates. This work was written in October 1995 but has never been published before taking the current form of a technical report. The history can be traced as follows. This work was derived from [Hu96].