GYO reductions, canonical connections, tree and cyclic schemas, and tree projections
Journal of Computer and System Sciences
Solving queries by tree projections
ACM Transactions on Database Systems (TODS)
Journal of the ACM (JACM)
On the Desirability of Acyclic Database Schemes
Journal of the ACM (JACM)
Degrees of acyclicity for hypergraphs and relational database schemes
Journal of the ACM (JACM)
The complexity of acyclic conjunctive queries
Journal of the ACM (JACM)
Foundations of Databases: The Logical Level
Foundations of Databases: The Logical Level
Constraint Satisfaction, Bounded Treewidth, and Finite-Variable Logics
CP '02 Proceedings of the 8th International Conference on Principles and Practice of Constraint Programming
Robbers, marshals, and guards: game theoretic and logical characterizations of hypertree width
Journal of Computer and System Sciences - Special issu on PODS 2001
Constraint solving via fractional edge covers
SODA '06 Proceedings of the seventeenth annual ACM-SIAM symposium on Discrete algorithm
Proceedings of the twenty-fifth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
The complexity of homomorphism and constraint satisfaction problems seen from the other side
Journal of the ACM (JACM)
Algorithms for acyclic database schemes
VLDB '81 Proceedings of the seventh international conference on Very Large Data Bases - Volume 7
Hypertree width and related hypergraph invariants
European Journal of Combinatorics
Efficient core computation in data exchange
Journal of the ACM (JACM)
Tree-Related Widths of Graphs and Hypergraphs
SIAM Journal on Discrete Mathematics
A unified theory of structural tractability for constraint satisfaction problems
Journal of Computer and System Sciences
Undirected connectivity in log-space
Journal of the ACM (JACM)
Tree Projections: Hypergraph Games and Minimality
ICALP '08 Proceedings of the 35th international colloquium on Automata, Languages and Programming, Part I
Approximating fractional hypertree width
SODA '09 Proceedings of the twentieth Annual ACM-SIAM Symposium on Discrete Algorithms
Generalized hypertree decompositions: NP-hardness and tractable variants
Journal of the ACM (JACM)
ICALP'07 Proceedings of the 34th international conference on Automata, Languages and Programming
Structural tractability of enumerating CSP solutions
CP'10 Proceedings of the 16th international conference on Principles and practice of constraint programming
Structural tractability of constraint optimization
CP'11 Proceedings of the 17th international conference on Principles and practice of constraint programming
Static analysis and optimization of semantic web queries
PODS '12 Proceedings of the 31st symposium on Principles of Database Systems
Static analysis and optimization of semantic web queries
ACM Transactions on Database Systems (TODS) - Invited papers issue
Tractable Hypergraph Properties for Constraint Satisfaction and Conjunctive Queries
Journal of the ACM (JACM)
Theoretical Computer Science
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Enforcing local consistency is a well-known technique to simplify the evaluation of conjunctive queries. It consists of repeatedly taking the semijion between every pair of (relations associated with) query atoms, until the procedure stabilizes. If some relation becomes empty, then the query has an empty answer. Otherwise, we cannot say anything in general, unless we have some information on the structure of the given query. In fact, a fundamental result in database theory states that the class of queries for which---on every database---local consistency entails global consistency is precisely the class of acyclic queries. In the last few years, several efforts have been made to define structural decomposition methods isolating larger classes of nearly-acyclic queries, yet retaining the same nice properties as acyclic ones. In particular, it is known that queries having bounded (generalized) hypertree-width can be evaluated in polynomial time, and that this structural property is also sufficient to guarantee that local consistency solves the problem, as for acyclic queries. However, the precise power of such an approach was an open problem: Is it the case that bounded generalized hypertree-width is also a necessary condition to guarantee that local consistency entails global consistency? In this paper, we positively answer this question, and go beyond. Firstly, we precisely characterize the power of local consistency procedures in the more general framework of tree projections, where a query Q and a set V of views (i.e., resources that can be used to answer Q) are given, and where one looks for an acyclic hypergraph covering Q and covered by Q---all known structural decomposition methods are just special cases of this framework, defining their specific set of resources. We show that the existence of tree projections of certain subqueries is a necessary and sufficient condition to guarantee that local consistency entails global consistency. In particular, tight characterizations are given not only for the decision problem, but also when answers restricted to variables covered by some view have to be computed. Secondly, we consider greedy tree-projections that are easy to compute, and we study how far they can be from arbitrary tree-projections, which are intractable in general. Finally, we investigate classes of instances not included in those having tree projections, and which can be easily recognized and define either new islands of tractability, or islands of quasi-tractability.