Rewriting of regular expressions and regular path queries

Abstract

Recent work on semi-structured data has revitalized the interest in path queries, i.e., queries that ask for all pairs of objects in the database that are connected by a path conforming to a certain specification, in particular to a regular expression. Also, in semi-structured data, as well as in data integration, data warehousing, and query optimization, the problem of query rewriting using views is receiving much attention: Given a query and a collection of views, generate a new query which uses the views and provides the answer to the original one. In this paper we address the problem of query rewriting using views in the context of semi-structured data. We present a method for computing the rewriting of a regular expression E in terms of other regular expressions. The method computes the exact rewriting (the one that defines the same regular language as E) if it exists, or the rewriting that defines the maximal language contained in the one defined by E, otherwise. We present a complexity analysis of both the problem and the method, showing that the latter is essentially optimal. Finally, we illustrate how to exploit the method to rewrite regular path queries using views in semi-structured data. The complexity results established for the rewriting of regular expressions apply also to the case of regular path queries.