A. Y. Levy. "Answering Queries Using Views: A Survey". VLDB Journal, 2001.  Home/Search   Document Details and Download   Summary   Related Articles   Check

....as follows: Contact: 949 824 9470 (tel) 949 824 4056 (fax) given a query on a database schema and a set of views over the same schema, can we answer the query using only the answers to the views Most of the recent work has addressed the problem when both queries and views are conjunctive. See [22] for a good survey. In most commercial scenarios, users require the exibility to pose queries using conjunctive queries along with arithmetic comparisons (e.g. between variables and constants that can take any value from a dense domain (e.g. real numbers) Similarly, views are also ....

A. Levy. Answering queries using views: A survey. Technical report, Computer Science Dept., Washington Univ., 2000.

....described by dictionary ( nite function) operations. Dictionaries also describe many physical access structures giving us succinct declarative descriptions of query plans, in the same language as queries. While sound and complete for the important case of path conjunctive materialized views [9, 16], the C B technique is sound for a larger class of queries, physical structures and constraints. We describe here the performance of a rst prototype that uses path conjunctive query graphs internally. The optimizations on which we concentrate here are increasingly relevant as more queries are ....

....that reduce the size of either the query or the constraint set by partitioning them into subparts that can be dealt with independently, in a dynamic programming style. Both strategies work well in common situations and one of them is complete for the case of path conjunctive materialized views [9, 16]. 2. We nd the technique very valuable when only the presence of semantic integrity constraints enables the use of physical access structures or materialized views. This situation clearly justi es the original intuition for this research direction [9, 19] Experiments We have built a prototype ....

[Article contains additional citation context not shown here]

A. Levy. Answering Queries Using Views: A Survey. Forthcoming.

....data integration, conjunctive query, builtin predicate, query containment, view, contained rewriting. 1 Introduction The problem of rewriting queries using views (aka query folding [Qia96] is of great importance in data integration, query optimization and physical data independence maintenance [Lev00]. For example, in a mediated data integration system, users are usually presented with a uniform interface through which queries are to be submitted. The uniform interface, also called the global schema, consists of a set of virtual relations (aka base relations) which may not be physically ....

....HS93] The rewritings we obtain are unions of general conjunctive queries. Our attention is focused on how to find all possible rewritings quickly, rather than on how e#cient the rewritings can be evaluated. There has been intensive research on the problem of rewriting queries using views, see [Lev00] for a survey. Among previous work on finding contained rewritings, the Bucket al..gorithm [LRO96] is used for conjunctive queries and conjunctive views where the views and the query may contain comparison predicates such as x a, y x. The Inverse rule algorithm [DG97b, DGL00] was proposed for ....

A. Levy. Answering queries using views: a survey. Technical report, Computer Science Dept, Washington Univ., 2000.

....on special signi cance. The problem can be stated as follows: given a query on a database schema and a set of views over the same schema, can we answer the query using only the answers to the views Most of the recent work has addressed the problem when both queries and views are conjunctive. See [23] for a good survey. In most commercial scenarios, users require the exibility to pose queries using conjunctive queries along with arithmetic comparisons (e.g. between variables and constants that can take any value from a dense domain (e.g. real numbers) Similarly, views are also ....

A. Levy. Answering queries using views: A survey. Technical report, Computer Science Dept., Washington Univ., 2000.

....with query rewriting using views are [3] and [4] in which equivalent rewritings of conjunctive queries are discussed. Over the past few years, the problem has received intensive attention mainly because of its relevance to data integration and query optimization. For a comprehensive survey, see [5]. Here we only mention a few papers that are closely related to our work. Among the early algorithms, the U join algorithm [1] and the Bucket al..gorithm [6] are used to find rewritings of conjunctive queries using conjunctive views (in the Bucket al..gorithm, the views and the query may contain ....

....There are two world assumptions [12] under the closed world assumption, the view relation stores all of the answers to the view; under the open world assumption, the view relation stores possibly only part of the answers to the view. The open world assumption is usually used in data integration [5] because it is usually not known that all answers of the views are actually stored. In this paper, we will use the open world assumption. For any base instance D consisting of instances of the base relations, we use W (D) to denote a view instance (with respect to D) consisting of instances ....

A. Levy. Answering queries using views: a survey. Technical report, Computer Science Dept, Washington Univ., 2000.

.... different ontologies, suitable query languages should be added to the ontology specification language, and considered in the various reasoning tasks, in the spirit of [4, 5] As a result query processing in this setting is strongly related to view based query answering in data integration systems [20, 17]. What distinguishes ontology integration from data integration as studied in databases, is that, while in data integration one assumes that each source is basically a databases, i.e. a logical theory with a single model, such an assumption is not made in ontology integration, where a local ....

....more powerful mechanism. In particular, such a mechanism should allow for mapping a concept in one ontology into a view, i.e. a query over the other ontologies, which acquires the relevant information by navigating and aggregating several concepts. Following the research done in data integration [16, 17], we can distinguish two basic approaches for defining this mapping: the global centric approach, where concepts of the global ontology G are mapped into queries over the local ontologies in S; the local centric approach, where concepts of the local ontologies in S are mapped to queries over ....

[Article contains additional citation context not shown here]

A. Y. Levy. Answering queries using views: A survey. Technical report, University of Washinghton, 1999.

....data integration, conjunctive query, builtin predicate, query containment, view, contained rewriting. 1 Introduction The problem of rewriting queries using views (aka query folding [Qia96] is of great importance in data integration, query optimization and physical data independence maintenance [Lev00]. For example, in a mediated data integration system, users are usually presented with a uniform interface through which queries are to be submitted. The uniform interface, also called the global schema, consists of a set of virtual relations (aka base relations) which may not be physically ....

....HS93] The rewritings we obtain are unions of general conjunctive queries. Our attention is focused on how to find all possible rewritings quickly, rather than on how e#cient the rewritings can be evaluated. There has been intensive research on the problem of rewriting queries using views, see [Lev00] for a survey. Among previous work on finding contained rewritings, the Bucket al..gorithm [LRO96] is used for conjunctive queries and conjunctive views where the views and the query may contain comparison predicates such as x a, y #= x. The Inverse rule algorithm [DG97b, DGL00] was proposed for ....

A. Levy. Answering queries using views: a survey. Technical report, Computer Science Dept, Washington Univ., 2000.

.... different ontologies, suitable query languages should be added to the ontology specification language, and considered in the various reasoning tasks, in the spirit of [4, 5] As a result query processing in this setting is strongly related to view based query answering in data integration systems [20, 17]. What distinguishes ontology integration from data integration as studied in databases, is that, while in data integration one assumes that each source is basically a databases, i.e. a logical theory with a single model, such an assumption is not made in ontology integration, where a local ....

....more powerful mechanism. In particular, such a mechanism should allow for mapping a concept in one ontology into a view, i.e. a query over the other ontologies, which acquires the relevant information by navigating and aggregating several concepts. Following the research done in data integration [16, 17], we can distinguish two basic approaches for defining this mapping: the global centric approach, where concepts of the global ontology G are mapped into queries over the local ontologies in S; the local centric approach, where concepts of the local ontologies in S are mapped to ....

[Article contains additional citation context not shown here]

A. Y. Levy. Answering queries using views: A survey. Technical report, University of Washinghton, 1999.

....several algorithms have been developed for finding rewritings of queries using views. Algorithms most closely related to our approach include the bucket al..gorithm [12, 17] the inverse rule algorithm [9, 21, 2] the MiniCon algorithm [20] and the Shared Variable Bucket al..gorithm [19] See [15] for a survey. These algorithms aim at generating contained rewritings for a query that compute a subset of the answer to the query, while we want to find equivalent rewritings that compute the same answer to a query. Another difference is that they have no optimization considerations since under ....

A. Levy. Answering queries using views: A survey. Technical report, Computer Science Dept., Washington Univ., 2000.

....path (sub)queries is very important. This optimizer can be used for the broader class of full fledged query languages for semistructured data. In semistructured data, as well as in other data models such as relational and object oriented, the importance of utilizing views is well recognized. [LMSS95, CGLV99, Lev99]. Simply stated, the problem is: Given a query Q and a set of views fV 1 ; V n g, find a representation of Q by means of the views and then answer the query on the basis of this representation. Several papers investigate this problem for the case of conjunctive queries [LMSS95, Ull97, ....

....cost of accessing the views. Depending on the application (information integration, cache based optimization, etc) different assumptions are valid. The use of rewritings in answering user queries using views have been thoroughly investigated in the case of relational databases (see e.g. the survey [Lev99]) For the case of semi structured databases much less is currently known. Notably, Calvanese et al. [CGLV99] show how to obtain l rewritings, and the same authors, in [CGLV2000] discuss the possible use of l rewritings in information integration applications. The present authors show in [GT2000] ....

A. Y. Levy. Answering queries using views: a survey. Submitted for publication 1999. 14

.... queries using views over monadic database schemas Jan Van den Bussche Limburg University, Belgium 1 Introduction In research in databases and AI, the problem of rewriting queries using views has received a lot of interest recently; we refer to the nice survey by Levy for background [Lev99]. In this short note, we o er two remarks on the computational complexity of this problem. First, we o er a most simple proof that the problem is NP complete. We should specify immediately that we work with the contained rewriting variant of the notion of rewriting queries using views. For the ....

....in general is undecidable, but is decidable if the database schema is monadic [BGG97] We show that over monadic database schemas, the problem of rewriting queries using views is eciently solvable. 2 Preliminaries We begin by recalling the de nitions of the needed notions. See the references [AHV95, Lev99, Ull89] for more background. A database schema S is a nite set of relation names, where each relation name has an associated natural number, called its arity. 1 An atomic formula over S is an expression of the form R( x) where R 2 S, and x is a k tuple of variables, where k is the arity of R. A ....

A.Y. Levy. Answering queries using views: A survey. Computer Science Department, University of Washington, 1999.

....problems, such as information integration, data warehousing, and query optimization. The problem can be stated as follows: given a query on a database schema and a set of views over the same schema, can we answer the query using only the answers to the views Recently, Levy compiled a good survey [16] about the different approaches to this problem. In the context of query optimization, computing a query using previously materialized views can speed up query processing, because part of the computation necessary for the query may have been done while computing the views. In a data warehouse, ....

A. Levy. Answering queries using views: A survey. Technical report, Computer Science Dept., Washington Univ., 2000.

....problem that have been considered vary depending on the languages used for describing the queries and the views. The rewriting problem that has been extensively studied concerns the pure relational setting where queries and views are expressed as conjunctive queries over relational atoms (see [7] for a recent survey) The first extension to DL has been considered in [2] which studied several instances of the problem of rewriting queries using views when the queries and the views are conjunctive queries over DL expressions. 8] deals with the problem of rewriting (union of) conjunctive ....

Alon Levy. Answering Queries Using Views: a Survey. To appear.

....problem that have been considered vary depending on the languages used for describing the queries and the views. The rewriting problem that has been extensively studied concerns the pure relational setting where queries and views are expressed as conjunctive queries over relational atoms (see [6] for a recent survey) The first extension to description logics (DL) has been considered in [2] which studied several instances of the problem of rewriting queries using views when the queries and the views are conjunctive queries over DL expressions. 7] deals with the problem of rewriting ....

Alon Levy. Answering Queries Using Views: a Survey. To appear.

....point an edge labelled article, followed by any number of other edges then by an edge ref and finally by an edge labelled with ullman or widom. In semistructured data, as well as in data integration, data warehousing and query optimization the problem of query rewriting using views is well known [20, 25, 9, 19]. Simply stated, the problem is: Given a query Q and a set of views fv1 ; vng, find a representation of Q by means of the views and then answer the query on the basis of this representation. Several papers investigate this problem for the case of conjunctive queries [20, 25, 11, 24] Their ....

....the number of input pairs is not considered by Calvanese et al. Instead they briefly discuss the possibility of using rewritings of regular queries in answering queries using views. Since rewritings have proved to be highly successful in attacking the corresponding problem for relational databases [19], one might hope that the same technique could be used for semistructured databases. Indeed, when the exact rewriting of a query Q using V exists, Calvanese et al. show that, under the exact view assumption the rewriting can be used to answer Q using S. Unfortunately, under the more realistic ....

A. Y. Levy. Answering queries using views: a survey. Submitted for publication 1999.

....information exchange. While XML has clear advantages as a description format, state of the art query optimization and query processing algorithms for data integration still rely on the relational model. This is the case, for example, of the existing algorithms for answering queries using views [9]. Given the richness of the semistructured model (and the peculiarities of XML as a data model) algorithms of equivalent efficiency and ease of use, but designed for XML, are more difficult to find. Recent projects like [1] and [4] put XML in the center of query processing, describing data ....

.... patterns to the native XML data, as well as to the actual access path to relational data (that was referred to under its XML interface in the query) We use a simple rewriting query using views algorithm which produces equivalent rewritings of the relational query, with respect to bag semantics[9]. Rewriting the previous query using the view V thus defined would yield: Result(patRec) Document(docID, V(docID,patRec, patient rec ) ElemContent(e1,e2,null, V(docID,e2, pers rec ) contains(e2, calcium ,depth,tag) contains(e2, deficiency ,depth,tag) 1 We use Datalog instead of ....

A. Y. Levy. Answering queries using views: a survey. submitted to publication, available at http://www.cs.washington.edu/homes/alon/.

....problems, such as information integration, data warehousing, and query optimization. The problem can be stated as follows: given a query on a database schema and a set of views over the same schema, can we answer the query using only the answers to the views Recently, Levy compiled a good survey [Lev00] about the di erent approaches to this problem. In the context of query optimization, computing a query using previously materialized views can speed up query processing, because part of the computation necessary for the query may have been done while computing the views. In a data warehouse, ....

....given, and queries can be arbitrary. We would like to deduce a minimal subset of views that can answer all queries answerable by the original set. 2 Background In this section, we review some concepts about answering queries using views [LMSS95] We assume the reader is familiar with the survey [Lev00] Let r 1 ; r m be m base relations in a database. We rst consider queries on the database in the following conjunctive form: 2 h( X) g 1 ( X 1 ) g k ( X k ) In each subgoal g i ( X i ) predicate g i is a base relation, and every argument in the subgoal is ....

Alon Y. Levy. Answering queries using views: A survey. In http://www.cs.washington.edu/homes/alon/site/files/view-survey.ps, 2000.

....described by dictionary ( nite function) operations. Dictionaries also describe many physical access structures giving us succinct declarative descriptions of query plans, in the same language as queries. While sound and complete for the important case of path conjunctive materialized views [11, 22], the C B technique is in fact sound for a much larger class of queries, physical structures and constraints. We describe here the performance of a rst prototype that uses path conjunctive query graphs internally. Extensions are possible and planned. We believe that the optimizations on which we ....

....and the size of the constraint set. But we have designed several strati cation strategies that make the backchase phase ecient and very worthwhile even for quite challenging queries. Moreover, one of these strategies is complete for the important case of path conjunctive materialized views [11, 22] just like the general technique. 2. We nd the technique very valuable when only the presence of semantic integrity constraints enables the use of physical access structures or materialized views. This situation clearly justi es the original intuition for this research direction [11, 27] ....

[Article contains additional citation context not shown here]

A. Levy. Answering Queries Using Views: A Survey. Forthcoming.

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A. Y. Levy. "Answering Queries Using Views: A Survey". VLDB Journal, 2001.

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A.Y. Levy. Answering queries using views: A survey. Technical report, University of Washinghton, 1999.

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A. Levy. Answering queries using views: A survey. Technical report, Computer Science Dept., Washington University, 2000.

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A. Y. Levy. "Answering Queries Using Views: A Survey". VLDB Journal, 2001.

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A. Levy. Answering Queries Using Views: A Survey. Forthcoming.

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