WC study the complexity of the problem of answering queries using materinlized views, This problem has attracted a lot of attention recently because of its relevance in data integration.Previous work considered only conjunctive view definitions.We examine the consequences of allowing more expressive view definition languages.Tl~olanguagcsweconsiderforviewdefinitionsanduserqueriesare: conjunctive qucrics with inequality, positive queries, datalog, and first-order logic.We show that the complexity of the problem depcnds on whether views are assumed to store all the tuples that satisfy the view definition, or only a subset of it.Finally, we apply the results to the view consistency and view self-maintainability problems which nrise in data warehousing.

We consider the problem of answering datalog queries using materialized views.The abiity to answer queries using views is crucial in the context of information integration.Previous work on answering queries using views restricted queries to being conjunctive.We extend this work to general recursive queries:Given a datalog program P and a set of views, is it possible to find a datalog program that is equivalent to P and only uses views as EDB predicates?In this paper, we show that the problem of whether a datalog program can be rewritten into an equivalent program that only uses views is undecidable.On the other hand, we prove that a datalog program P can be effectively rewritten into a program that only uses views, that is contained in P, and that contains all programs that only use views and are contained in P. As a consequence, if there exists a program equivalent to 'P that only uses views, then our construction is guaranteed to yield a program equivalent to P.

Infomaster is an information integration system that provides integrated access to multiple distributed heterogeneous information sources on the Internet, thus giving the illusion of a centralized, homogeneous information system. We say that Infomaster creates a virtual data warehouse. The core of Infomaster is a facilitator that dynamically determines an efficient way to answer the user's query using as few sources as necessary and harmonizes the heterogeneities among these sources. Infomaster handles both structural and content translation to resolve differences between multiple data sources and the multiple applications for the collected data. Infomaster connects to a variety of databases using wrappers, such as for Z39.50, SQL databases through ODBC, EDI transactions, and other World Wide Web (WWW) sources. There are several WWW user interfaces to Infomaster, including forms based and textual. Infomaster also includes a programmatic interface and it can download results in structured form onto a client computer. Infomaster has been in production use for integrating rental housing advertisements from several newspapers (since fall 1995), and for meeting room scheduling (since winter 1996). Infomaster is also being used to integrate heterogeneous electronic product catalogs.