Results 1  10
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61
A comparison of structural CSP decomposition methods
 Artificial Intelligence
, 2000
"... We compare tractable classes of constraint satisfaction problems (CSPs). We first give a uniform presentation of the major structural CSP decomposition methods. We then introduce a new class of tractable CSPs based on the concept of hypertree decomposition recently developed in Database Theory. We i ..."
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Cited by 149 (17 self)
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We compare tractable classes of constraint satisfaction problems (CSPs). We first give a uniform presentation of the major structural CSP decomposition methods. We then introduce a new class of tractable CSPs based on the concept of hypertree decomposition recently developed in Database Theory. We introduce a framework for comparing parametric decompositionbased methods according to tractability criteria and compare the most relevant methods. We show that the method of hypertree decomposition dominates the others in the case of general (nonbinary) CSPs.
Semantics and Complexity of SPARQL
"... SPARQL is the standard language for querying RDF data. In this article, we address systematically the formal study of the database aspects of SPARQL, concentrating in its graph pattern matching facility. We provide a compositional semantics for the core part of SPARQL, and study the complexity of th ..."
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Cited by 134 (13 self)
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SPARQL is the standard language for querying RDF data. In this article, we address systematically the formal study of the database aspects of SPARQL, concentrating in its graph pattern matching facility. We provide a compositional semantics for the core part of SPARQL, and study the complexity of the evaluation of several fragments of the language. Among other complexity results, we show that the evaluation of general SPARQL patterns is PSPACEcomplete. We identify a large class of SPARQL patterns, defined by imposing a simple and natural syntactic restriction, where the query evaluation problem can be solved more efficiently. This restriction gives rise to the class of welldesigned patterns. We show that the evaluation problem is coNPcomplete for welldesigned patterns. Moreover, we provide several rewriting rules for welldesigned patterns whose application may have a considerable impact in the cost of evaluating SPARQL queries.
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 132 (13 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.
Pure Nash Equilibria: Hard and Easy Games
"... In this paper we investigate complexity issues related to pure Nash equilibria of strategic games. We show that, even in very restrictive settings, determining whether a game has a pure Nash Equilibrium is NPhard, while deciding whether a game has a strong Nash equilibrium is Stcomplete. We then s ..."
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Cited by 63 (2 self)
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In this paper we investigate complexity issues related to pure Nash equilibria of strategic games. We show that, even in very restrictive settings, determining whether a game has a pure Nash Equilibrium is NPhard, while deciding whether a game has a strong Nash equilibrium is Stcomplete. We then study practically relevant restrictions that lower the complexity. In particular, we are interested in quantitative and qualitative restrictions of the way each player's move depends on moves of other players. We say that a game has small neighborhood if the " utility function for each player depends only on (the actions of) a logarithmically small number of other players, The dependency structure of a game G can he expressed by a graph G(G) or by a hypergraph II(G). Among other results, we show that if jC has small neighborhood and if II(G) has botmdecl hypertree width (or if G(G) has bounded treewidth), then finding pure Nash and Pareto equilibria is feasible in polynomial time. If the game is graphical, then these problems are LOGCFLcomplete and thus in the class _NC ~ of highly parallelizable problems. 1 Introduction and Overview of Results The theory of strategic games and Nash equilibria has important applications in economics and decision making [31, 2]. Determining whether Nash equilibria exist, and effectively computing
Specifying and reasoning about dynamic accesscontrol policies
 of Lecture Notes in Computer Science
, 2006
"... Abstract. Accesscontrol policies have grown from simple matrices to nontrivial specifications written in sophisticated languages. The increasing complexity of these policies demands correspondingly strong automated reasoning techniques for understanding and debugging them. The need for these techn ..."
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Cited by 37 (3 self)
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Abstract. Accesscontrol policies have grown from simple matrices to nontrivial specifications written in sophisticated languages. The increasing complexity of these policies demands correspondingly strong automated reasoning techniques for understanding and debugging them. The need for these techniques is even more pressing given the rich and dynamic nature of the environments in which these policies evaluate. We define a framework to represent the behavior of accesscontrol policies in a dynamic environment. We then specify several interesting, decidable analyses using firstorder temporal logic. Our work illustrates the subtle interplay between logical and statebased methods, particularly in the presence of threevalued policies. We also define a notion of policy equivalence that is especially useful for modular reasoning. 1
Xpath leashed
 IN ACM COMPUTING SURVEYS
, 2007
"... This survey gives an overview of formal results on the XML query language XPath. We identify several important fragments of XPath, focusing on subsets of XPath 1.0. We then give results on the expressiveness of XPath and its fragments compared to other formalisms for querying trees, algorithms and c ..."
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Cited by 36 (3 self)
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This survey gives an overview of formal results on the XML query language XPath. We identify several important fragments of XPath, focusing on subsets of XPath 1.0. We then give results on the expressiveness of XPath and its fragments compared to other formalisms for querying trees, algorithms and complexity bounds for evaluation of XPath queries, and static analysis of XPath queries.
Logics for unranked trees: an overview
 Logical Methods in Computer Science 2, Issue 3, Paper 2
, 2006
"... Vol. 2 (3:2) 2006, pp. 1–31 www.lmcsonline.org ..."
Hypertree decompositions: A survey
 In: MFCS ’01: Proceedings of the 26th International Symposium on Mathematical Foundations of Computer Science
, 2001
"... Abstract. This paper surveys recent results related to the concept of hypertree decomposition and the associated notion of hypertree width. A hypertree decomposition of a hypergraph (similar to a tree decomposition of a graph) is a suitable clustering of its hyperedges yielding a tree or a forest. I ..."
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Cited by 29 (4 self)
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Abstract. This paper surveys recent results related to the concept of hypertree decomposition and the associated notion of hypertree width. A hypertree decomposition of a hypergraph (similar to a tree decomposition of a graph) is a suitable clustering of its hyperedges yielding a tree or a forest. Important NP hard problems become tractable if restricted to instances whose associated hypergraphs are of bounded hypertree width. We also review a number of complexity results on problems whose structure is described by acyclic or nearly acyclic hypergraphs. 1
When is the evaluation of conjunctive queries tractable
 In Proceedings of the 33rd ACM Symposium on Theory of Computing
, 2001
"... zINRIARocquencourt Abstract The evaluation of conjunctive queries is hard both with respect to its combined complexity (NPcomplete) and its parameterized complexity (W[1]complete). It becomes tractable (PTIME for combined complexity, FPT for parameterized complexity), when the underlying graphs o ..."
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Cited by 26 (1 self)
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zINRIARocquencourt Abstract The evaluation of conjunctive queries is hard both with respect to its combined complexity (NPcomplete) and its parameterized complexity (W[1]complete). It becomes tractable (PTIME for combined complexity, FPT for parameterized complexity), when the underlying graphs of the conjunctive queries have bounded treewidth [2]. We show that, in some sense, this is optimal both with respect to combined and parameterized complexity: For every class C of graphs, the evaluation of all conjunctive queries whose underlying graph is in C is tractable if, and only if, C has bounded treewidth. A technical result of independent interest is that the colored grid homomorphism problem is NPcomplete and, if parameterized by the grid size, W[1]complete. 1. Introduction Conjunctive queries are relational database queries expressed by formulas of firstorder logic that are of the form
Constraint Satisfaction and Database Theory: a Tutorial
 19th ACM Symposium on Principles of Database Systems
, 2000
"... A large class of problems in AI and other areas of computer science can be viewed as constraintsatisfaction problems. This includes problems in machine vision, belief maintenance, scheduling, temporal reasoning, type reconstruction, graph theory, and satisability. In general, the constraint satisfa ..."
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Cited by 20 (0 self)
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A large class of problems in AI and other areas of computer science can be viewed as constraintsatisfaction problems. This includes problems in machine vision, belief maintenance, scheduling, temporal reasoning, type reconstruction, graph theory, and satisability. In general, the constraint satisfactionproblem is NPcomplete, so searching for tractable cases is an active research area. It turns out that constraint satisfaction has an intimate connection with database theory: constraintsatisfaction problems can be recast as database problems and database problems can be recast as constraintsatisfaction problems. In this tutorial, I will cover the fundamentals of constraints saisfaction and describe its intimate relationship with database theory from various perspectives. 1 Introduction Since the early 1970s, researchers in articial intelligence have investigated a class of combinatorial problems that became known as constraintsatisfaction problems (CSP). The input to such a pro...