Alon Y. Levy

Database techniques for the World-Wide Web: a survey

The popularity of the World-Wide Web (WWW) has made it a prime vehicle for disseminating information. The relevance of database concepts to the problems of managing and querying this information has led to a signi cant body of recent research addressing these problems. Even though the underlying challenge is the one that has been traditionally addressed by the database community { how to manage large volumes of data { the novel context of the WWW forces us to signi cantly extend previous techniques. The primary goal of this survey is to classify the di erent tasks to which database concepts have been applied, and to emphasize the technical innovations that were required to do so.

An adaptive query execution system for data integration

Query processing in data integration occurs over network-bound, autonomous data sources. This requires extensions to traditional optimization and execution techniques for three reasons: there is an absence of quality statistics about the data, data transfer rates are unpredictable and bursty, and slow or unavailable data sources can often be replaced by overlapping or mirrored sources. This paper presents the Tukwila data integration system, designed to support adaptivity at its core using a two-pronged approach. Interleaved planning and execution with partial optimization allows Tukwila to quickly recover from decisions based on inaccurate estimates. During execution, Tukwila uses adaptive query operators such as the double pipelined hash join, which produces answers quickly, and the dynamic collector, which robustly and efficiently computes unions across overlapping data sources. We demonstrate that the Tukwila architecture extends previous innovations in adaptive execution (such as query scrambling, mid-execution re-optimization, and choose nodes), and we present experimental evidence that our techniques result in behavior desirable for a data integration system.

Combining Horn rules and description logics in CARIN

Abstract We describe carin , a novel family of representation languages, that combine the expressive power of Horn rules and of description logics. We address the issue of providing sound and complete inference procedures for such languages. We identify existential entailment as a core problem in reasoning in carin , and describe an existential entailment algorithm for the ALCNR description logic. As a result, we obtain a sound and complete algorithm for reasoning in non-recursive carin ALCNR knowledge bases, and an algorithm for rule subsumption over ALCNR . We show that in general, the reasoning problem for recursive carin - ALCNR knowledge bases is undecidable, and identify the constructors of ALCNR causing the undecidability. We show two ways in which carin - ALCNR knowledge bases can be restricted while obtaining sound and complete reasoning.

Answering queries using views (extended abstract)

We consider the problem of computing answers to queries by using materialized views. Aside from its potential in optimizing query evaluation, the problem also arises in applications such as Global Information Systems, Mobile Computing and maintaining physical data independence. We consider the problem of finding a rewriting of a query that uses the materialized views, the problem of finding minimal rewritings, and finding complete rewritings (i.e., rewritings that use only the views). We show that all the possible rewritings can be obtained by considering cent ainment mappings from the views to the query, and that the problems we consider are NP-complete when both the query and the views are conjunctive and don’t involve builtin comparison predicates. We show that the problem has two independent sources of complexity (the number of possible containment mappings, and the complexity of deciding which literals from the original query can be deleted). We describe a polynomial time algorithm for finding rewritings, and show that under certain conditions, it will find the minimal rewriting. Finally, we analyze the complexity of the problems when the queries and views may be disjunctive and involve built-in comparison predicates.

Data model and query evaluation in global information systems

Global information systems involve a large number of information sources distributed over computer networks. The variety of information sources and disparity of interfaces makes the task of easily locating and efficiently accessing information over the network very cumbersome. We describe an architecture for global information systems that is especially tailored to address the challenges raised in such an environment, and distinguish our architecture from architectures of multidatabase and distributed database systems. Our architecture is based on presenting a conceptually unified view of the information space to a user, specifying rich descriptions of the contents of the information sources, and using these descriptions for optimizing queries posed in the unified view. The contributions of this paper include: (1) we identify aspects of site descriptions that are useful in query optimization; (2) we describe query optimization techniques that minimize the number of information sources accessed; and (3) we demonstrate the need for interleaving planning and query execution in such a system, and present an algorithm for this purpose.

logic-based techniques in data integration

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 researches 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 languages for describing contents of data sources, the tradeoffs between them, and the associated reformulation algorithms.

Recursive query plans for data integration

Abstract Generating query-answering plans for data integration systems requires to translate a user query, formulated in terms of a mediated schema, to a query that uses relations that are actually stored in data sources. Previous solutions to the translation problem produced sets of conjunctive plans , and were therefore limited in their ability to handle recursive queries and to exploit data sources with binding-pattern limitations and functional dependencies that are known to hold in the mediated schema. As a result, these plans were incomplete w.r.t. sources encountered in practice (i.e., produced only a subset of the possible answers). We describe the novel class of recursive query answering plans, which enables us to settle three open problems. First, we describe an algorithm for finding a query plan that produces the maximal set of answers from the sources for arbitrary recursive queries. Second, we extend this algorithm to use the presence of functional and full dependencies in the mediated schema. Third, we describe an algorithm for finding the maximal query plan in the presence of binding-pattern restrictions in the sources. In all three cases, recursive plans are necessary in order to obtain a maximal query plan.

A query language for a Web-site management system

We have designed a system, called STRUDEL, which applies familiar concepts from database management systems, to the process of building web sites. The main motivation for developing STRUDEL is the observation that with current technology, creating and managing large sites is tedious, because a site designer must simultaneously perform (at least) three tasks: (1) choosing what information will be available at the site, (2) organizing that information in individual pages or in graphs of linked pages, and (3) specifying the visual presentation of pages in HTML. Furthermore, since there is no separation between the physical organization of the information underlying a web site and the logical view we have on it, changing or restructuring a site are unwieldy tasks. In STRUDEL, the web site manager can separate the logical view of information available at a web site, the structure of that information in linked pages, and the graphical presentation of pages in HTML. First, the site builder defines independently the data that will be available at the site. This process may require creating an integrated view of data from multiple (external) sources. Second, the site builder defines the structure of the web-site. The structure is defined as a view over the underlying information, and different versions of the site can be defined by specifying multiple views. Finally, the graphical representation of the pages in the web site is specified. This paper describes the query language that lies at the heart of the STRUDEL system. In STRUDEL, we model the da ta at the different levels as graphs. That is, the data in the external sources, the da ta in the integrated view and the web-site itself are modeled as graphs. A graph model is appropriate because site da ta may be derived from multiple sources, such as existing database systems and HTML files. Consequently, our system requires a query language for (1) defining the integrated view of the data, and (2) defining the structure of web sites. An important requirement of our query language is that it be able to construct graphs. Our query processor needs to be able to answer queries tha t involve accessing different da ta sources. Even though we model the sources as containing graphs, we cannot assume they have a uniform representation of graphs. Hence, our query processor needs to adhere to possible limitations on access to data in the graphs, and should be able to exploit additional querying capabilities that an external source may have. We have designed a general framework for processing STRUDEL queries over multiple unstructured data sources, and are designing optimizations that use the capabilities of external sources whenever possible. The purpose of this paper is to describe the syntax and semantics of STRUQL, the query language at the core of STRUDEL. We believe that STRuQL is a language of independent interest, and is useful for other applications involving the management of semistructured data, as well as a view definition language for such data. We discuss the relationship of STRUQL to other languages proposed in the li terature in Section 6: see [Abi97, Bun97].

Query optimization in the presence of limited access patterns

We consider the problem of query optimization in the presence of limitations on access patterns to the data (i.e., when one must provide values for one of the attributes of a relation in order to obtain tuples). We show that in the presence of limited access patterns we must search a space of annotated query plans, where the annotations describe the inputs that must be given to the plan. We describe a theoretical and experimental analysis of the resulting search space and a novel query optimization algorithm that is designed to perform well under the different conditions that may arise. The algorithm searches the set of annotated query plans, pruning invalid and non-viable plans as early as possible in the search space, and it also uses a best-first search strategy in order to produce a first complete plan early in the search. We describe experiments to illustrate the performance of our algorithm.

Query containment for conjunctive queries with regular expressions

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.