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127
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 142 (15 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.
Identifying the minimal transversals of a hypergraph and related problems
 SIAM Journal on Computing
, 1995
"... The paper considers two decision problems on hypergraphs, hypergraph saturation and recognition of the transversal hypergraph, and discusses their significance for several search problems in applied computer science. Hypergraph saturation, i.e., given a hypergraph H, decide if every subset of vertic ..."
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Cited by 135 (7 self)
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The paper considers two decision problems on hypergraphs, hypergraph saturation and recognition of the transversal hypergraph, and discusses their significance for several search problems in applied computer science. Hypergraph saturation, i.e., given a hypergraph H, decide if every subset of vertices is contained in or contains some edge of H, is shown to be coNPcomplete. A certain subproblem of hypergraph saturation, the saturation of simple hypergraphs, is shown to be computationally equivalent to transversal hypergraph recognition, i.e., given two hypergraphs H 1; H 2, decide if the sets in H 2 are all the minimal transversals of H 1. The complexity of the search problem related to the recognition of the transversal hypergraph, the computation of the transversal hypergraph, is an open problem. This task needs time exponential in the input size, but it is unknown whether an outputpolynomial algorithm exists for this problem. For several important subcases, for instance if an upper or lower bound is imposed on the edge size or for acyclic hypergraphs, we present outputpolynomial algorithms. Computing or recognizing the minimal transversals of a hypergraph is a frequent problem in practice, which is pointed out by identifying important applications in database theory, Boolean switching theory, logic, and AI, particularly in modelbased diagnosis.
Mining Minimal NonRedundant Association Rules using Frequent Closed Itemsets
, 2000
"... The problem of the relevance and the usefulness of extracted association rules is of primary importance because, in the majority of cases, reallife databases lead to several thousands association rules with high condence and among which are many redundancies. Using the closure of the Galois con ..."
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Cited by 88 (6 self)
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The problem of the relevance and the usefulness of extracted association rules is of primary importance because, in the majority of cases, reallife databases lead to several thousands association rules with high condence and among which are many redundancies. Using the closure of the Galois connection, we dene two new bases for association rules which union is a generating set for all valid association rules with support and condence. These bases are characterized using frequent closed itemsets and their generators; they consist of the nonredundant exact and approximate association rules having minimal antecedents and maximal consequents, i.e. the most relevant association rules. Algorithms for extracting these bases are presented and results of experiments carried out on reallife databases show that the proposed bases are useful, and that their generation is not time consuming.
Fuzzy functional dependencies and lossless join decomposition of fuzzy relational database systems
 ACM Transactions on Database Systems
, 1988
"... This paper deals with the application of fuzzy logic in a relational database environment with the objective of capturing more meaning of the data. It is shown that with suitable interpretations for the fuzzy membership functions, a fuzzy relational data model can be used to represent ambiguities in ..."
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Cited by 78 (0 self)
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This paper deals with the application of fuzzy logic in a relational database environment with the objective of capturing more meaning of the data. It is shown that with suitable interpretations for the fuzzy membership functions, a fuzzy relational data model can be used to represent ambiguities in data values as well as impreciseness in the association among them. Relational operators for fuzzy relations have been studied, and applicability of fuzzy logic in capturing integrity constraints has been investigated. By introducing a fuzzy resemblance measure EQUAL for comparing domain values, the definition of classical functional dependency has been generalized to fuzzy functional dependency (ffd). The implication problem of ffds has been examined and a set of sound and complete inference axioms has been proposed. Next, the problem of lossless join decomposition of fuzzy relations for a given set of fuzzy functional dependencies is investigated. It is proved that with a suitable restriction on EQUAL, the design theory of a classical relational database with functional dependencies can be extended to fuzzy relations satisfying fuzzy functional dependencies.
Monadic Datalog and the Expressive Power of Languages for Web Information Extraction
 J. ACM
, 2002
"... Research on information extraction from Web pages (wrapping) has seen much activity in recent times (particularly systems implementations), but little work has been done on formally studying the expressiveness of the formalisms proposed or on the theoretical foundations of wrapping. In this paper, w ..."
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Cited by 77 (11 self)
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Research on information extraction from Web pages (wrapping) has seen much activity in recent times (particularly systems implementations), but little work has been done on formally studying the expressiveness of the formalisms proposed or on the theoretical foundations of wrapping. In this paper, we first study monadic datalog as a wrapping language (over ranked or unranked tree structures). Using previous work by Neven and Schwentick, we show that this simple language is equivalent to full monadic second order logic (MSO) in its ability to specify wrappers. We believe that MSO has the right expressiveness required for Web information extraction and thus propose MSO as a yardstick for evaluating and comparing wrappers. Using the above result, we study the kernel fragment Elog of the Elog wrapping language used in the Lixto system (a visual wrapper generator). The striking fact here is that Elog exactly captures MSO, yet is easier to use. Indeed, programs in this language can be entirely visually specified. We also formally compare Elog to other wrapping languages proposed in the literature.
Efficient Evaluation of XML Middleware Queries
, 2001
"... We address the problem of e#ciently constructing materialized XML views of relational databases. In our setting, the XML view is specified by a query in the declarative query language of a middleware system, called SilkRoute. The middleware system evaluates a query by sending one or more SQL queri ..."
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Cited by 73 (5 self)
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We address the problem of e#ciently constructing materialized XML views of relational databases. In our setting, the XML view is specified by a query in the declarative query language of a middleware system, called SilkRoute. The middleware system evaluates a query by sending one or more SQL queries to the target relational database, integrating the resulting tuple streams, and adding the XML tags. We focus on how to best choose the SQL queries, without having control over the target RDBMS. 1.
Horn clauses and database dependencies
 Journal of the ACM
, 1982
"... Abstract. Certain firstorder sentences, called "dependencies, " about relations in a database are defined and studied. These dependencies seem to include all prewously defined dependencies as special cases A new concept is mtroduced, called "faithfulness (with respect to ..."
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Cited by 68 (6 self)
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Abstract. Certain firstorder sentences, called &quot;dependencies, &quot; about relations in a database are defined and studied. These dependencies seem to include all prewously defined dependencies as special cases A new concept is mtroduced, called &quot;faithfulness (with respect to direct product), &quot; which enables powerful results to be proved about the existence of &quot;Armstrong relations &quot; in the presence of these new dependencies. (An Armstrong relaUon is a relation that obeys precisely those dependencies that are the logical consequences of a given set of dependencies.) Results are also obtained about characterizing the class of projections of those relations that obey a given set of dependencies.
Tractable Constraints in Finite Semilattices
 Science of Computer Programming
, 1996
"... . We introduce the notion of definite inequality constraints involving monotone functions in a finite meetsemilattice, generalizing the logical notion of Hornclauses, and we give a linear time algorithm for deciding satisfiability. We characterize the expressiveness of the framework of definit ..."
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Cited by 56 (3 self)
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. We introduce the notion of definite inequality constraints involving monotone functions in a finite meetsemilattice, generalizing the logical notion of Hornclauses, and we give a linear time algorithm for deciding satisfiability. We characterize the expressiveness of the framework of definite constraints and show that the algorithm uniformly solves exactly the set of all meetclosed relational constraint problems, running with small linear time constant factors for any fixed problem. We give an alternative technique which reduces inequalities to satisfiability of Hornclauses (hornsat) and study its efficiency. Finally, we show that the algorithm is complete for a maximal class of tractable constraints, by proving that any strict extension will lead to NPhard problems in any meetsemilattice. Keywords: Finite semilattices, constraint satisfiability, program analysis, tractability, algorithms. 1 Introduction Many program analysis problems can be solved by generating a...
Transformational design and implementation of a new efficient solution to the ready simulation problem
 Science of Computer Programming
, 1995
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On the structure of Armstrong relations for functional dependencies
 Journal of the ACM
, 1984
"... Abstract. An Armstrong relation for a set of functional dependencies (FDs) is a relation that satisfies each FD implied by the set but no FD that is not implied by it. The structure and size (number of tuples) of Armstrong relatsons are investigated. Upper and lower bounds on the size of minimalsiz ..."
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Cited by 45 (3 self)
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Abstract. An Armstrong relation for a set of functional dependencies (FDs) is a relation that satisfies each FD implied by the set but no FD that is not implied by it. The structure and size (number of tuples) of Armstrong relatsons are investigated. Upper and lower bounds on the size of minimalsized Armstrong relations are derived, and upper and lower bounds on the number of distinct entries that must appear m an Armstrong relation are given. It is shown that the time complexity of finding an Armstrong relation, gwen a set of functional dependencies, is precisely exponential in the number of attributes. Also shown,s the falsity of a natural conjecture which says that almost all relations obeying a given set of FDs are Armstrong relations for that set of FDs. Finally, Armstrong relations are used to generahze a result, obtained by Demetrovics using quite complicated methods, about the possible sets of keys for a relauon.