Distributed data processing is fast becoming a reality. Businesses want to have it for many reasons, and they often must have it in order to stay competitive. While much of the infrastructure for distributed data processing is already in place (e.g., modern network technology), there are a number of issues which still make distributed data processing a complex undertaking: (1) distributed systems can become very large involving thousands of heterogeneous sites including PCs and mainframe server machines � (2) the state of a distributed system changes rapidly because the load of sites varies over time and new sites are added to the system� (3) legacy systems need to be integrated|such legacy systems usually have not been designed for distributed data processing and now need to interact with other (modern) systems in a distributed environment. This paper presents the state of the art of query processing for distributed database and information systems. The paper presents the \textbook " architecture for distributed query processing and a series of techniques that are particularly useful for distributed database systems. These techniques include special join techniques, techniques to exploit intra-query parallelism, techniques to reduce communication costs, and techniques to exploit caching and replication of data. Furthermore, the paper discusses di erent kinds of distributed systems such as client-server, middleware (multi-tier), and heterogeneous database systems and shows how query processing works in these systems. Categories and subject descriptors: E.5 [Data]:Files � H.2.4 [Database Management Systems]: distributed databases, query processing � H.2.5 [Heterogeneous Databases]: data translation General terms: algorithms � performance Additional key words and phrases: query optimization � query execution � client-server databases � middleware � multi-tier architectures � database application systems � wrappers� replication � caching � economic models for query processing � dissemination-based information systems 1

The data integration problem is to provide uniform access to multiple heterogeneous information sources available online (e.g., databases on the WWW). This problem has recently received considerable attention from researchers in the fields of Artificial Intelligence and Database Systems. The data integration problem is complicated by the facts that (1) sources contain closely related and overlapping data, (2) data is stored in multiple data models and schemas, and (3) data sources have differing query processing capabilities. A key element in a data integration system is the language used to describe the contents and capabilities of the data sources. While such a language needs to be as expressive as possible, it should also enable to efficiently address the main inference problem that arises in this context: to translate a user query that is formulated over a mediated schema into a query on the local schemas. This paper describes several lanaguages for describing contents of data sources, ...

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Abstract. This paper deals with some modeling aspects that have to be addressed in the context of the integration of heterogeneous and autonomous XML resources. We propose an integration system, but the emphasis of this paper is neither on its algorithmic aspects nor on its technical details. Instead, we focus on the significance of offering appropriate high-level primitives and mechanisms for representing the semantics of XML data. We posit that support for such primitives and mechanisms is a pre-requisite for realizing the goals of the semantic Web. 1

Abstract. There are basically two approaches for designing a data integration system. In the global-as-view (GAV) approach, one maps the concepts in the global schema to views over the sources, whereas in the local-as-view (LAV) approach, one maps the sources into views over the global schema. The goal of this paper is to relate the two approaches with respect to their expressive power. The analysis is carried out in a relational database setting, where both the queries on the global schema, and the views in the mapping are conjunctive queries. We introduce the notion of query-preserving transformation, and query-reducibility between data integration systems, and we show that, when no integrity constraints are allowed in global schema, the LAV and the GAV approaches are incomparable. We then consider the addition of integrity constraints in the global schema, and present techniques for query-preserving transformations in both directions. Finally, we show that our results imply that we can always transform any system following the GLAV approach (a generalization of both LAV and GAV) into a query-preserving GAV system. 1

Abstract. The increasing popularity of XML Web services motivates us to examine if it is feasible to substitute one vendor service for another when using a Web-based application, assuming that these services are “derived from ” a common base. If such substitution were possible, end users could use the same application with a variety of back-end vendor services, and the vendors themselves could compete on price, quality, availability, etc. Interoperability with substituted services is non-trivial, however, and four types of incompatibilities may arise during such interoperation – structural, value, encoding and semantic. We address these incompatibilities three-fold: (1) static and dynamic analysis tools to infer whether an application is compatible with a substituted service, (2) semiautomatically generated middleware components called cross-stubs that actually resolve incompatibilities and enable interoperation with substituted services, and (3) a lightweight mechanism called multi-option types to enable applications to be written from the ground up in an interoperation-friendly manner. Using real applications and services as examples, we both demonstrate and evaluate our tools and techniques for enabling interoperation with substituted services. 1

We consider the problem of answering queries using views, where queries and views are conjunctive queries with arithmetic comparisons (CQACs) over dense orders. Previous work only considered limited variants of this problem, without giving a complete solution. We have developed an ecient algorithm to obtain maximally contained rewritings (MCRs) for queries having left (or right) semi-interval-comparison predicates. For semi-interval queries we show that at least recursive datalog is necessary to nd a maximally-contained solution, and identify cases where datalog is sucient. Finally, we show that obtaining equivalent rewritings for CQAC's is decidable

We study the problem of generating efficient, equivalent rewritings using views to compute the answer to a query. We take the closed-world assumption, in which views are materialized from base relations, rather than views describing sources in terms of abstract predicates, as is common when the open-world assumption is used. In the closedworld model, there can be an infinite number of different rewritings that compute the same answer, yet have quite different performance. Query optimizers take a logical plan (a rewriting of the query) as an input, and generate efficient physical plans to compute the answer. Thus our goal is to generate a small subset of the possible logical plans without missing an optimal physical plan.

This paper considers Peer-to-Peer systems in which peers employ taxonomies for describing the contents of their objects and for formulating semantic-based queries to the other peers of the system. As each peer can use its own taxonomy, peers are equipped with inter-taxonomy mappings in order to carry out the required translation tasks. As these systems are ad-hoc, the peers should be able to create or revise these mappings on demand and at run-time. For this reason, we introduce an ostensive data-driven method for automatic mapping and specialize it for the case of taxonomies.

In this paper, we investigate a first step towards the long-term vision of the Semantic Web by studying the problem of answering queries posed through a mediated ontology to multiple information sources whose content is described as views over the ontology relations. The contributions of this paper are twofold. We first o#er a uniform logical setting which allows us to encompass and to relate the existing work on answering and rewriting queries using views. In particular, we make clearer the connection between the problem of rewriting queries using views and the problem of answering queries using extensions of views. Then we focus on an instance of the problem of rewriting conjunctive queries using views through an ontology expressed in a description logic, for which we exhibit a complete algorithm