Query containment for conjunctive queries with regular expressions

Abstract

The management of semistructured data has recently rccoivcd significant attention because of the need of several applications to model and query large volumes of irregular data. This paper considers the problem of query containment for a query language over semistructured data, STRUQLO, that contains the essential feature common to all such languages, namely the ability to specify regular path expressions over the data. We show hcrc that containment of STRUQLO queries is decidable. First, we give a semantic criterion for STRUQLO query containment: WC show that it suffices to check containment on only finitely many canonical databases. Second, we give a syntactic criteria for query containment, based on a notion of query mappings, which extends containment mappings for conjunctive queries. Third, wc consider a certain fragment of STRUQLO, obtained by imposing restrictions on the regular path expressions, and show that query containment for this fragment of STRUQLO is NP complete.