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117
Answering Queries Using Views: A Survey
, 2000
"... The problem of answering queries using views is to find efficient methods of answering a query using a set of previously defined materialized views over the database, rather than accessing the database relations. The problem has recently received significant attention because of its relevance to a w ..."
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Cited by 562 (32 self)
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The problem of answering queries using views is to find efficient methods of answering a query using a set of previously defined materialized views over the database, rather than accessing the database relations. The problem has recently received significant attention because of its relevance to a wide variety of data management problems. In query optimization, finding a rewriting of a query using a set of materialized views can yield a more efficient query execution plan. To support the separation of the logical and physical views of data, a storage schema can be described using views over the logical schema. As a result, finding a query execution plan that accesses the storage amounts to solving the problem of answering queries using views. Finally, the problem arises in data integration systems, where data sources can be described as precomputed views over a mediated schema. This article surveys the state of the art on the problem of answering queries using views, and synthesizes the disparate works into a coherent framework. We describe the different applications of the problem, the algorithms proposed to solve it and the relevant theoretical results.
On the Decidability of Query Containment under Constraints
"... Query containment under constraints is the problem of checking whether for every database satisfying a given set of constraints, the result of one query is a subset of the result of another query. Recent research points out that this is a central problem in several database applications, and we addr ..."
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Cited by 256 (56 self)
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Query containment under constraints is the problem of checking whether for every database satisfying a given set of constraints, the result of one query is a subset of the result of another query. Recent research points out that this is a central problem in several database applications, and we address it within a setting where constraints are specified in the form of special inclusion dependencies over complex expressions, built by using intersection and difference of relations, special forms of quantification, regular expressions over binary relations, and cardinality constraints. These types of constraints capture a great variety of data models, including the relational, the entityrelational, and the objectoriented model. We study the problem of checking whether q is contained in q ′ with respect to the constraints specified in a schema S, where q and q ′ are nonrecursive Datalog programs whose atoms are complex expressions. We present the following results on query containment. For the case where q does not contain regular expressions, we provide a method for deciding query containment, and analyze its computational complexity. We do the same for the case where neither S nor q, q ′ contain number restrictions. To the best of our knowledge, this yields the first decidability result on containment of conjunctive queries with regular expressions. Finally, we prove that the problem is undecidable for the case where we admit inequalities in q′.
HYPERTREE DECOMPOSITIONS AND TRACTABLE QUERIES
, 1998
"... Several important decision problems on conjunctive queries (CQs) are NPcomplete in general but become tractable, and actually highly parallelizable, if restricted to acyclic or nearly acyclic queries. Examples are the evaluation of Boolean CQs and query containment. These problems were shown tracta ..."
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Cited by 165 (42 self)
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Several important decision problems on conjunctive queries (CQs) are NPcomplete in general but become tractable, and actually highly parallelizable, if restricted to acyclic or nearly acyclic queries. Examples are the evaluation of Boolean CQs and query containment. These problems were shown tractable for conjunctive queries of bounded treewidth [7], and of bounded degree of cyclicity [18, 17]. The so far most general concept of nearly acyclic queries was the notion of queries of bounded querywidth introduced by Chekuri and Rajaraman [7]. While CQs of bounded query width are tractable, it remained unclear whether such queries are efficiently recognizable. Chekuri and Rajaraman [7] stated as an open problem whether for each constant k it can be determined in polynomial time if a query has query width ≤ k. We give a negative answer by proving this problem NPcomplete (specifically, for k = 4). In order to circumvent this difficulty, we introduce the new concept of hypertree decomposition of a query and the corresponding notion of hypertree width. We prove: (a) for each k, the class of queries with query width bounded by k is properly contained in the class of queries whose hypertree width is bounded by k; (b) unlike query width, constant hypertreewidth is efficiently recognizable; (c) Boolean queries of constant hypertree width can be efficiently evaluated.
ConjunctiveQuery Containment and Constraint Satisfaction
 Journal of Computer and System Sciences
, 1998
"... Conjunctivequery containment is recognized as a fundamental problem in database query evaluation and optimization. At the same time, constraint satisfaction is recognized as a fundamental problem in artificial intelligence. What do conjunctivequery containment and constraint satisfaction have in c ..."
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Cited by 164 (14 self)
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Conjunctivequery containment is recognized as a fundamental problem in database query evaluation and optimization. At the same time, constraint satisfaction is recognized as a fundamental problem in artificial intelligence. What do conjunctivequery containment and constraint satisfaction have in common? Our main conceptual contribution in this paper is to point out that, despite their very different formulation, conjunctivequery containment and constraint satisfaction are essentially the same problem. The reason is that they can be recast as the following fundamental algebraic problem: given two finite relational structures A and B, is there a homomorphism h : A ! B? As formulated above, the homomorphism problem is uniform in the sense that both relational structures A and B are part of the input. By fixing the structure B, one obtains the following nonuniform problem: given a finite relational structure A, is there a homomorphism h : A ! B? In general, nonuniform tractability results do not uniformize. Thus, it is natural to ask: which tractable cases of nonuniform tractability results for constraint satisfaction and conjunctivequery containment do uniformize? Our main technical contribution in this paper is to show that several cases of tractable nonuniform constraint satisfaction problems do indeed uniformize. We exhibit three nonuniform tractability results that uniformize and, thus, give rise to polynomialtime solvable cases of constraint satisfaction and conjunctivequery containment.
Towards a Theory of Natural Language Interfaces to Databases
, 2003
"... The need for N a ur a La ngua' Interfa3; to da ta ba ses (NLIs) ha s become increa3] glya cute a more a d more peoplea ccess informa ion through their web browsers, PDAs, a d cell phones.Yet NLIsa re only usa ble if theyma p na tura la gu a. questions to SQL queries correctly As Schneiderma a ..."
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Cited by 142 (3 self)
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The need for N a ur a La ngua' Interfa3; to da ta ba ses (NLIs) ha s become increa3] glya cute a more a d more peoplea ccess informa ion through their web browsers, PDAs, a d cell phones.Yet NLIsa re only usa ble if theyma p na tura la gu a. questions to SQL queries correctly As Schneiderma a d Norma ha vea rgued, people a. unwilling to tr a e relia lea nd predicta ble user interfaMM for intelligent but unrelia ble ones In thispa per, we introducea theoretica l fra mework for relia ble NLIs, which is the founda tion for the fully implemented Precise NLI We prove tha t, fora broa cla3 of semantically tractable tura la gu a. questions, Precise is gua.' teed to ma p e a h question to the corresponding SQL query We report on experiments testing Precise on severa l hundred questions dra wn from user studies over three benchma rkda ta ba ses We findtha t over 80% of the questionsa' sem a tica;M traE a ble questions, which Precise a swers correctly lly recognizes the 20% of questions tha t it c a not h a dle, a d requestsa pa ra phra se Fina lly, we show tha t Precise compa res fa vora ly with Mooney'slea: ing NLIa nd with Microsoft 's English Query product Categories and Subject Descriptors H 2 3 [Database Management]: Query L a gua.E3 SQL; H 5 2 [Information Interfaces and Presentation]: User Interfa:M , N a ur a La ngua' General Terms gua: Algorithms, Relia ility Keywords tura la gu a. interfa'H da ta ba se, relia ility # We th a k Keith Golden, D a Weld, Bonnie Weber a d Tessa La for comments on previous dra' s Thisresea] h wa s supported in pa' by ONRgra t N000140210324 Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for prof...
Conjunctive query answering for the description logic SHIQ
, 2007
"... Conjunctive queries play an important role as an expressive query language for Description Logics (DLs). Although modern DLs usually provide for transitive roles, it was an open problem whether conjunctive query answering over DL knowledge bases is decidable if transitive roles are admitted in the q ..."
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Cited by 140 (28 self)
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Conjunctive queries play an important role as an expressive query language for Description Logics (DLs). Although modern DLs usually provide for transitive roles, it was an open problem whether conjunctive query answering over DL knowledge bases is decidable if transitive roles are admitted in the query. In this paper, we consider conjunctive queries over knowledge bases formulated in the popular DL SHIQ and allow transitive roles in both the query and the knowledge base. We show that query answering is decidable and establish the following complexity bounds: regarding combined complexity, we devise a deterministic algorithm for query answering that needs time single exponential in the size of the KB and double exponential in the size of the query. Regarding data complexity, we prove coNPcompleteness. 1
Composing Mappings among Data Sources
 In VLDB
, 2003
"... Semantic mappings between data sources play a key role in several data sharing architectures. Mappings provide the relationships between data stored in different sources, and therefore enable answering queries that require data from other nodes in a data sharing network. Composing mappings is one of ..."
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Cited by 140 (9 self)
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Semantic mappings between data sources play a key role in several data sharing architectures. Mappings provide the relationships between data stored in different sources, and therefore enable answering queries that require data from other nodes in a data sharing network. Composing mappings is one of the core problems that lies at the heart of several optimization methods in data sharing networks, such as caching frequently traversed paths and redundancy analysis.
Foundations of Semantic Web Databases
 IN: PODS ’04: PROCEEDINGS OF THE TWENTYTHIRD ACM SIGMODSIGACTSIGART SYMPOSIUM ON PRINCIPLES OF DATABASE SYSTEMS
, 2004
"... The Semantic Web is based on the idea of adding more machinereadable semantics to web information via annotations written in a language called the Resource Description Framework (RDF). RDF resembles a subset of binary firstorder logic including the ability to refer to anonymous objects. Its extend ..."
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Cited by 113 (22 self)
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The Semantic Web is based on the idea of adding more machinereadable semantics to web information via annotations written in a language called the Resource Description Framework (RDF). RDF resembles a subset of binary firstorder logic including the ability to refer to anonymous objects. Its extended version, RDFS, supports reification, typing and inheritance. These features introduce new challenges into the formal study of sets of RDF/RDFS statements and languages for querying them. Although several such query languages have been proposed, there has been little work on foundational aspects. We investigate these, including computational aspects of testing entailment and redundancy. We propose a query language with welldefined semantics and study the complexity of query processing, query containment, and simplification of answers.
Query Evaluation via TreeDecompositions
 JOURNAL OF THE ACM
, 2001
"... A number of efficient methods for evaluating firstorder and monadicsecond order queries on finite relational structures are based on treedecompositions of structures or queries. We systematically study these methods. In the first part of the paper we consider arbitrary formulas on treelike struc ..."
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Cited by 105 (16 self)
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A number of efficient methods for evaluating firstorder and monadicsecond order queries on finite relational structures are based on treedecompositions of structures or queries. We systematically study these methods. In the first part of the paper we consider arbitrary formulas on treelike structures. We generalize a theorem of Courcelle [8] by showing that on structures of bounded treewidth a monadic secondorder formula (with free first and secondorder variables) can be evaluated in time linear in the structure size plus the size of the output. In the second part we study treelike formulas on arbitrary structures. We generalize the notions of acyclicity and bounded treewidth from conjunctive queries to arbitrary firstorder formulas in a straightforward way and analyze the complexity of evaluating formulas of these fragments. Moreover, we show that the acyclic and bounded treewidth fragments have the same expressive power as the wellknown guarded fragment and the finitevariable fragments of firstorder logic, respectively.