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11
Optimizing Large Join Queries in Mediation Systems
 International Conference on Database Theory (ICDT
, 1999
"... . In data integration systems, queries posed to a mediator need to be translated into a sequence of queries to the underlying data sources. In a heterogeneous environment, with sources of diverse and limited query capabilities, not all the translations are feasible. In this paper, we study the probl ..."
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Cited by 40 (11 self)
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. In data integration systems, queries posed to a mediator need to be translated into a sequence of queries to the underlying data sources. In a heterogeneous environment, with sources of diverse and limited query capabilities, not all the translations are feasible. In this paper, we study the problem of finding feasible and efficient query plans for mediator systems. We consider conjunctive queries on mediators and model the source capabilities through attributebinding adornments. We use a simple cost model that focuses on the major costs in mediation systems, those involved with sending queries to sources and getting answers back. Under this metric, we develop two algorithms for source query sequencing  one based on a simple greedy strategy and another based on a partitioning scheme. The first algorithm produces optimal plans in some scenarios, and we show a linear bound on its worst case performance when it misses optimal plans. The second algorithm generates optimal plans in mor...
On the Complexity of Dataflow Analysis of Logic Programs
, 1992
"... This article reports some results on this correlation in the context of logic programs. A formal notion of the "precision" of an analysis algorithm is proposed, and this is used to characterize the worstcase computational complexity of a number of dataflow analyses with different degrees ..."
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Cited by 35 (4 self)
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This article reports some results on this correlation in the context of logic programs. A formal notion of the "precision" of an analysis algorithm is proposed, and this is used to characterize the worstcase computational complexity of a number of dataflow analyses with different degrees of precision. While this article considers the analysis of logic programs, the technique proposed, namely the use of "exactness sets" to study relationships between complexity and precision of analyses, is not specific to logic programming in any way, and is equally applicable to flow analyses of other language families.
Computing Complete Answers to Queries in the Presence of Limited Access Patterns
 Journal of VLDB
, 1999
"... In data applications such as information integration, there can be limited access patterns to relations, i.e., binding patterns require values to be specified for certain attributes in order to retrieve data from a relation. As a consequence, we cannot retrieve all tuples from these relations. In th ..."
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Cited by 23 (3 self)
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In data applications such as information integration, there can be limited access patterns to relations, i.e., binding patterns require values to be specified for certain attributes in order to retrieve data from a relation. As a consequence, we cannot retrieve all tuples from these relations. In this article we study the problem of computing the complete answer to a query, i.e., the answer that could be computed if all the tuples could be retrieved. A query is stable if for any instance of the relations in the query, its complete answer can be computed using the access patterns permitted by the relations. We study the problem of testing stability of various classes of queries, including conjunctive queries, unions of conjunctive queries, and conjunctive queries with arithmetic comparisons.
Processing Unions of Conjunctive Queries with Negation under Limited Access Patterns
 IN EDBT
, 2004
"... We study the problem of answering queries over sources with limited access patterns. The problem is to decide whether a given query Q is feasible, i.e., equivalent to an executable query Q # that observes the limited access patterns given by the sources. We characterize the complexity of decidin ..."
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Cited by 22 (8 self)
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We study the problem of answering queries over sources with limited access patterns. The problem is to decide whether a given query Q is feasible, i.e., equivalent to an executable query Q # that observes the limited access patterns given by the sources. We characterize the complexity of deciding feasibility for the classes CQ (conjunctive queries with negation) and UCQ (unions of CQ queries): Testing feasibility is just as hard as testing containment and therefore # 2 complete. We also provide a uniform treatment for CQ, UCQ, CQ , and UCQ by devising a single algorithm which is optimal for each of these classes. In addition, we show how one can often avoid the worstcase complexity by certain approximations: At compiletime, even if a query Q is not feasible, we can find e#ciently the minimal executable query containing Q. For query answering at runtime, we devise an algorithm which may report complete answers even in the case of infeasible plans and which can indicate to the user the degree of completeness for certain incomplete answers.
Processing FirstOrder Queries under Limited Access Patterns
 IN PODS
, 2004
"... We study the problem of answering queries over sources with limited access patterns. Given a firstorder query Q, the problem is to decide whether there is an equivalent query which can be executed observing the access patterns restrictions. If so, we say that Q is feasible. We define feasible for f ..."
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Cited by 18 (2 self)
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We study the problem of answering queries over sources with limited access patterns. Given a firstorder query Q, the problem is to decide whether there is an equivalent query which can be executed observing the access patterns restrictions. If so, we say that Q is feasible. We define feasible for firstorder queriesprevious definitions handled only some existential casesand characterize the complexity of many firstorder query classes. For each of them, we show that deciding feasibility is as hard as deciding containment. Since feasibility is undecidable in many cases and hard to decide in some others, we also define an approximation to it which can be computed in NP for any firstorder query and in P for unions of conjunctive queries with negation. Finally, we outline a practical overall strategy for processing firstorder queries under limited access patterns.
Efficiently ordering subgoals with access constraints
"... In this paper, we study the problem of ordering subgoals under binding pattern restrictions for queries posed as nonrecursive Datalog programs. We prove that despite their limited expressive power, the problem is computationally hard — PSPACEcomplete in the size of the nonrecursive Datalog program ..."
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Cited by 8 (0 self)
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In this paper, we study the problem of ordering subgoals under binding pattern restrictions for queries posed as nonrecursive Datalog programs. We prove that despite their limited expressive power, the problem is computationally hard — PSPACEcomplete in the size of the nonrecursive Datalog program even for fairly restricted cases. As a practical solution to this problem, we develop an asymptotically optimal algorithm that runs in time linear in the size of the query plan. We also study extensions of our algorithm that efficiently solve other query planning problems under binding pattern restrictions. These problems include conjunctive queries with nested grouping constraints, distributed conjunctive queries, and firstorder queries.
Topk Query Processing in Probabilistic Databases with NonMaterialized Views
, 2012
"... In this paper, we investigate a novel approach of computing confidence bounds for topk ranking queries in probabilistic databases with nonmaterialized views. Unlike prior approaches, we present an exact pruning algorithm for finding the topranked query answers according to their marginal probabil ..."
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Cited by 3 (2 self)
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In this paper, we investigate a novel approach of computing confidence bounds for topk ranking queries in probabilistic databases with nonmaterialized views. Unlike prior approaches, we present an exact pruning algorithm for finding the topranked query answers according to their marginal probabilities without the need to first materialize all answer candidates via the views. Specifically, we consider conjunctive queries over multiple levels of selectprojectjoin views, the latter of which are cast into Datalog rules, where also the rules themselves may be uncertain, i.e., be valid with some degree of confidence. To our knowledge, this work is the first to address integrated data and confidence computations in the context of probabilistic databases by considering confidence bounds over partially evaluated query answers with firstorder lineage formulas. We further extend our query processing techniques by a toolsuite of scheduling strategies based on selectivity estimation and the expected impact of subgoals on the final confidence of answer candidates. Experiments with large datasets demonstrate drastic runtime improvements over both sampling and decompositionbased methods—even
Computing Complete Answers to Queries with Binding Restrictions
, 1999
"... We consider the problem of computing the complete answer to a query when there is limited access to relations, i.e., when binding patterns require values to be specified for certain attributes in order to retrieve data from a relation. This problem is common in informationintegration systems, wh ..."
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Cited by 1 (1 self)
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We consider the problem of computing the complete answer to a query when there is limited access to relations, i.e., when binding patterns require values to be specified for certain attributes in order to retrieve data from a relation. This problem is common in informationintegration systems, where heterogeneous sources have diverse and limited query capabilities. A query is stable if for any instance of the relations mentioned in the query, the complete answer to the query can be computed, using only the binding patterns permitted for queries at the various sources. We first study conjunctive queries, and we show that a conjunctive query is stable if and only if its minimal equivalent query Qm has an order of all the subgoals in Qm , such that each subgoal in the order can be queried with a legal binding pattern. We propose two algorithms for testing stability of conjunctive queries, and we prove this problem is NPhard. For a nonstable conjunctive query, whether its comp...