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42
A filtering process for general constraintsatisfaction problems: Achieving pairewiseconsistency using an associated binary representation
 CRIM, University Montpellier II
, 1989
"... In this paper, we are interesting in using a partial consistency, issued from relational databases theory, within the ConstraintSatisfaction Problems (CSPs) framework: pairwiseconsistency. This partial consistency concerns general CSPs (i.e. CSPs the constraints of which may involve more than two v ..."
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Cited by 14 (0 self)
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In this paper, we are interesting in using a partial consistency, issued from relational databases theory, within the ConstraintSatisfaction Problems (CSPs) framework: pairwiseconsistency. This partial consistency concerns general CSPs (i.e. CSPs the constraints of which may involve more than two variables). We provide a polynomial algorithm for achieving it; then we can extend the class of polynomially solvable CSPs. This algorithm is based on a minimal binary representation of a general CSP, of which presents some properties we give. Key words: Coi'straintSatisfaction Problems (CSP), partial consistency, arcconsistency, pairwiseconsistency, qual graph. consistency, pairwiseconsistency, for general CSPs, and a polynomial algorithm to process it; this allows us to extend the class of polynomially solvable CSPs. The basic idea is to
Finding Minimum Height Elimination Trees For Interval Graphs In Polynomial Time
 BIT
, 1994
"... . The elimination tree plays an important role in many aspects of sparse matrix factorization. The height of the elimination tree presents a rough, but usually effective, measure of the time needed to perform parallel elimination. Finding orderings that produce low elimination trees is therefore im ..."
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Cited by 13 (2 self)
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. The elimination tree plays an important role in many aspects of sparse matrix factorization. The height of the elimination tree presents a rough, but usually effective, measure of the time needed to perform parallel elimination. Finding orderings that produce low elimination trees is therefore important. As the problem of finding minimum height elimination tree orderings is NPhard, it is interesting to concentrate on limited classes of graphs and find minimum height elimination trees for these efficiently. In this paper, we use clique trees to find an efficient algorithm for interval graphs which make an important subclass of chordal graphs. We first illustrate this method through an algorithm that finds minimum height elimination trees for chordal graphs. This algorithm, although of exponential time complexity, is conceptionally simple and leads to a polynomialtime algorithm for finding minimum height elimination trees for interval graphs. AMS subject classifications: 05C50, 65F...
Towards the Study of Performance Tradeoffs Between Materialized and Virtual Integrated Views
, 1996
"... Consider the problem of supporting an integrated view over multiple databases. The traditional approach is to use a virtual view, but recent investigations are proposing to use a materialized view, or a hybrid virtual/materialized view. This paper initiates an investigation into the performance trad ..."
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Cited by 11 (0 self)
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Consider the problem of supporting an integrated view over multiple databases. The traditional approach is to use a virtual view, but recent investigations are proposing to use a materialized view, or a hybrid virtual/materialized view. This paper initiates an investigation into the performance tradeoffs along this spectrum of choices. In particular, the paper develops analytical models for predicting queryresponse time, view freshness, and system load in terms of parameters such as query frequency, query complexity, update frequency, and network delay. In many ways the performance of a mediatorbased integration environment is similar to that of a clientserver DBMS architecture. However, the notion of query freshness does not explicitly arise in a single DBMS, and complex joins may more likely in an integration environment. This paper lays the groundwork for conducting benchmarking experiments for integrated views, and presents the results of some initial experimentation. Such resul...
YARS2: A Federated Repository for Searching and Querying Graph Structured Data. 2007
 Proceedings of the 6th International Semantic Web Conference, Busan, Korea
, 2007
"... Abstract. We present the architecture of an endtoend semantic search engine that uses a graph data model to enable interactive query answering over structured and interlinked data collected from many disparate sources on the Web. In particular, we study distributed indexing methods for graphstruct ..."
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Cited by 11 (2 self)
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Abstract. We present the architecture of an endtoend semantic search engine that uses a graph data model to enable interactive query answering over structured and interlinked data collected from many disparate sources on the Web. In particular, we study distributed indexing methods for graphstructured data and parallel query evaluation methods on a cluster of computers. We evaluate the system on a dataset with 430 million statements collected from the Web, and provide scaleup experiments on 7 billion synthetically generated statements. Copyright c ○ 2007 by the authors 2 DERI TR 20070528 1
Weighted hypertree decompositions and optimal query plans
 In Proc. of PODS’04
, 2004
"... Hypertree width [22, 25] is a measure of the degree of cyclicity of hypergraphs. A number of relevant problems from different areas, e.g., the evaluation of conjunctive queries in database theory or the constraint satisfaction in AI, are tractable when their underlying hypergraphs have bounded hyper ..."
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Cited by 8 (2 self)
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Hypertree width [22, 25] is a measure of the degree of cyclicity of hypergraphs. A number of relevant problems from different areas, e.g., the evaluation of conjunctive queries in database theory or the constraint satisfaction in AI, are tractable when their underlying hypergraphs have bounded hypertree width. However, in practical contexts like the evaluation of database queries, we have more information besides the structure of queries. For instance, we know the number of tuples in relations, the selectivity of attributes and so on. In fact, all commercial queryoptimizers are based on quantitative methods and do not care about structural properties. In this paper, we define the notion of weighted hypertree decomposition, in order to combine structural decomposition methods with quantitative approaches. Weighted hypertree decompositions are equipped with cost functions, that can be used for modelling many
Querying incomplete information in semistructured data
 Journal of Computer and System Sciences
"... Semistructured data occur in situations where information lacks a homogeneous structure and is incomplete. Yet, up to now the incompleteness of information has not been reflected by special features of query languages. Our goal is to investigate the principles of queries that allow for incomplete an ..."
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Cited by 7 (3 self)
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Semistructured data occur in situations where information lacks a homogeneous structure and is incomplete. Yet, up to now the incompleteness of information has not been reflected by special features of query languages. Our goal is to investigate the principles of queries that allow for incomplete answers. We do not present, however, a concrete query language. Queries over classical structured data models contain a number of variables and constraints on these variables. An answer is a binding of the variables by elements of the database such that the constraints are satisfied. In the present paper, we loosen this concept in so far as we allow also answers that are partial; that is, not all variables in the query are bound by such an answer. Partial answers make it necessary to refine the model of query evaluation. The first modification relates to the satisfaction of constraints: in some circumstances we consider constraints involving unbound variables as satisfied. Second, in order to prevent a proliferation of answers, we only accept answers that are maximal in the sense that there are no assignments that bind more variables and satisfy the constraints of the query. Our model of query evaluation consists of two phases, a search phase and a filter phase. Semistructured databases are essentially labeled directed graphs. In the search phase, we use a query graph containing variables to match a maximal portion of the database graph. We investigate three different semantics for query
Private communication
, 1995
"... Given a graph G and positive integers b and w, the blackandwhite coloring problem asks about the existence of a partial vertexcoloring of G, with b vertices colored black and w white, such that there is no edge between a black and a white vertex. We suggest an improved algorithm for solving this ..."
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Cited by 5 (0 self)
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Given a graph G and positive integers b and w, the blackandwhite coloring problem asks about the existence of a partial vertexcoloring of G, with b vertices colored black and w white, such that there is no edge between a black and a white vertex. We suggest an improved algorithm for solving this problem on trees. Submitted:
Distributed Evaluation of Network Directory Queries
 TKDE
, 2003
"... This paper describes novel efficient techniques for the distributed evaluation of hierarchical aggregate selection queries over LDAP directory data, distributed across multiple autonomous directory servers. Such queries are useful for emerging applications like the Directory Enabled Networks initi ..."
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Cited by 4 (0 self)
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This paper describes novel efficient techniques for the distributed evaluation of hierarchical aggregate selection queries over LDAP directory data, distributed across multiple autonomous directory servers. Such queries are useful for emerging applications like the Directory Enabled Networks initiative. Our techniques follow the LDAP approach of distributed query evaluation by referrals, where each relevant server computes answers locally and the LDAP client coordinates between directory servers. We make a conceptual separation between the identification of relevant servers and the distributed computation of answers. We focus on the challenging task of generating an efficient plan for evaluating hierarchical aggregate selection queries, which involves correlating directory entries across multiple servers. The key features of our plan are: 1) The network traffic consists of query answers and auxiliary messages that depend only on the number of servers and the size of the query (not on the data size), 2) the coordination effort at the client is independent of the data size, and 3) potentially expensive servertoserver communication and coordination is avoided.
The semijoin algebra and the guarded fragment
 J. Logic Lang. Inform
, 2005
"... The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We discuss some interesting connections between the semijoin algebra and the guarded fragment of firstorder logic. We also provide an EhrenfeuchtFraïssé game, characteriz ..."
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Cited by 4 (0 self)
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The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We discuss some interesting connections between the semijoin algebra and the guarded fragment of firstorder logic. We also provide an EhrenfeuchtFraïssé game, characterizing the discerning power of the semijoin algebra. This game gives a method for showing that certain queries are not expressible in the semijoin algebra. 1