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16
DBXplorer: A system for keywordbased search over relational databases
 In ICDE
, 2002
"... Internet search engines have popularized the keywordbased search paradigm. While traditional database management systems offer powerful query languages, they do not allow keywordbased search. In this paper, we discuss DBXplorer, a system that enables keywordbased search in relational databases. DBX ..."
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Cited by 234 (3 self)
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Internet search engines have popularized the keywordbased search paradigm. While traditional database management systems offer powerful query languages, they do not allow keywordbased search. In this paper, we discuss DBXplorer, a system that enables keywordbased search in relational databases. DBXplorer has been implemented using a commercial relational database and web server and allows users to interact via a browser frontend. We outline the challenges and discuss the implementation of our system including results of extensive experimental evaluation. 1.
Query optimization in database systems
 ACM Computing Surveys
, 1984
"... Efficient methods of processing unanticipated queries are a crucial prerequisite for the success of generalized database management systems. A wide variety of approaches to improve the performance of query evaluation algorithms have been proposed: logicbased and semantic transformations, fast imple ..."
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Cited by 207 (0 self)
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Efficient methods of processing unanticipated queries are a crucial prerequisite for the success of generalized database management systems. A wide variety of approaches to improve the performance of query evaluation algorithms have been proposed: logicbased and semantic transformations, fast implementations of basic operations, and combinatorial or heuristic algorithms for generating alternative access plans and choosing among them. These methods are presented in the framework of a general query evaluation procedure using the relational calculus representation of queries. In addition, nonstandard query optimization issues such as higher level query evaluation, query optimization in distributed databases, and use of database machines are addressed. The focus, however, is on query optimization in centralized database systems.
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 125 (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.
Conjunctive Query Containment Revisited
, 1998
"... We consider the problems of conjunctive query containment and minimization, which are known to be NPcomplete, and show that these problems can be solved in polynomial time for the class of acyclic queries. We then generalize the notion of acyclicity and define a parameter called query width that ca ..."
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Cited by 99 (0 self)
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We consider the problems of conjunctive query containment and minimization, which are known to be NPcomplete, and show that these problems can be solved in polynomial time for the class of acyclic queries. We then generalize the notion of acyclicity and define a parameter called query width that captures the "degree of cyclicity" of a query: in particular, a query is acyclic if and only if its query width is 1. We give algorithms for containment and minimization that run in time polynomial in n k , where n is the input size and k is the query width. These algorithms naturally generalize those for acyclic queries, and are of practical significance because many queries have small query width compared to their sizes. We show that good bounds on the query width of Q can be obtained using the treewidth of the incidence graph of Q. We then consider the problem of finding an equivalent query to a given conjunctive query Q that has the least number of subgoals. We show that a polynomial tim...
Query Processing in a System for Distributed Databases (SDD1
 ACM Transactions on Database Systems
, 1981
"... Thii paper describes the techniques used to optimize relational queries in the SDD1 distributed database system. Queries are submitted to SDD1 in a highlevel procedural language called Datalanguage. Optimization begins by translating each Datalanguage query into a relational calculus form called ..."
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Cited by 70 (0 self)
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Thii paper describes the techniques used to optimize relational queries in the SDD1 distributed database system. Queries are submitted to SDD1 in a highlevel procedural language called Datalanguage. Optimization begins by translating each Datalanguage query into a relational calculus form called an envelope, which is essentially an aggregatefree QUEL query. This paper is primarily concerned with the optimization of envelopes. Envelopes are processed in two phases. The first phase executes relational operations at various sites of the distributed database in order to delimit a subset of the database that contains all data relevant to the envelope. This subset is called a reduction of the database. The second phase transmits the reduction to one designated site, and the query is executed locally at that site. The critical optimization problem is to perform the reduction phase efficiently. Success depends on designing a good repertoire of operators to use during this phase, and an effective algorithm for deciding which of these operators to use in processing a given envelope against a given database. The principal reduction operator that we employ is called a
New Results on Monotone Dualization and Generating Hypergraph Transversals
 SIAM JOURNAL ON COMPUTING
, 2002
"... We consider the problem of dualizing a monotone CNF (equivalently, computing all minimal transversals of a hypergraph), whose associated decision problem is a prominent open problem in NPcompleteness. We present a number of new polynomial time resp. outputpolynomial time results for significant ..."
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Cited by 37 (12 self)
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We consider the problem of dualizing a monotone CNF (equivalently, computing all minimal transversals of a hypergraph), whose associated decision problem is a prominent open problem in NPcompleteness. We present a number of new polynomial time resp. outputpolynomial time results for significant cases, which largely advance the tractability frontier and improve on previous results. Furthermore, we show that duality of two monotone CNFs can be disproved with limited nondeterminism. More precisely, this is feasible in polynomial time with O(log² n/log log n) suitably guessed bits. This result sheds new light on the complexity of this important problem.
Necessary and sufficient conditions to linearize doubly recursive programs in logic databases
 ACM TRANS. DATABASE SYST
, 1990
"... Linearization of nonlinear recursive programs is an important issue in logic databases for both practical and theoretical reasons. If a nonlinear recursive program can be transformed into an equivalent linear recursive program, then it may be computed more efficiently than when the transformation is ..."
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Cited by 10 (0 self)
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Linearization of nonlinear recursive programs is an important issue in logic databases for both practical and theoretical reasons. If a nonlinear recursive program can be transformed into an equivalent linear recursive program, then it may be computed more efficiently than when the transformation is not possible. We provide a set of necessary and sufficient conditions for a simple doubly recursive program to be equivalent to a simple linear recursive program. The necessary and sufficient conditions can be verified effectively.
Generating relations from XML documents
 In ICDT
, 2003
"... Abstract. This paper discusses several mechanisms for creating relations out of XML documents. A relation generator consists of two parts: (1) a tuple of path expressions and (2) an index indicating which path expressions may not be assigned the null value. Evaluating a relation generator involves f ..."
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Cited by 9 (5 self)
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Abstract. This paper discusses several mechanisms for creating relations out of XML documents. A relation generator consists of two parts: (1) a tuple of path expressions and (2) an index indicating which path expressions may not be assigned the null value. Evaluating a relation generator involves finding tuples of nodes that satisfy the path expressions and are related to one another in a meaningful fashion. Different semantics for evaluation are given that take into account the possible presence of incomplete information. The complexity of generating relations from documents is analyzed and evaluation algorithms are described. 1
Graph Homotopy and Graham Homotopy
, 2000
"... Simplehomotopy for simplicial and CW complexes is a special kind of topological homotopy constructed by elementary collapses and expansions. In this paper we introduce graph homotopy for graphs and Graham homotopy for hypergraphs, and study the relation between these homotopies and the simplehomoto ..."
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Cited by 5 (0 self)
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Simplehomotopy for simplicial and CW complexes is a special kind of topological homotopy constructed by elementary collapses and expansions. In this paper we introduce graph homotopy for graphs and Graham homotopy for hypergraphs, and study the relation between these homotopies and the simplehomotopy for simplicial complexes. The graph homotopy is useful to describe topological properties of discretized geometric figures, while the Graham homotopy is essential to characterize acyclic hypergraphs and acyclic relational database schemes.
Tractable Counting of the Answers to Conjunctive Queries
"... Conjunctive queries (CQs) are one of the most fundamental forms of database queries. In general, the evaluation of CQs is NPcomplete. Consequently, there has been an intensive search for tractable fragments. In this paper, we want to initiate a systematic search for tractable fragments of the count ..."
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Cited by 3 (0 self)
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Conjunctive queries (CQs) are one of the most fundamental forms of database queries. In general, the evaluation of CQs is NPcomplete. Consequently, there has been an intensive search for tractable fragments. In this paper, we want to initiate a systematic search for tractable fragments of the counting problem of CQs, i.e., the problem of counting the answers to a CQ. We prove several new tractability and intractability results by starting with acyclic conjunctive queries and generalising these results to CQs of bounded hypertreewidth.