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Views and queries: Determinacy and rewriting
 ACM Trans. Database Syst
"... We investigate the question of whether a query Q can be answered using a set V of views. We first define the problem in informationtheoretic terms: we say that V determines Q if V provides enough information to uniquely determine the answer to Q. Next, we look at the problem of rewriting Q in term ..."
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We investigate the question of whether a query Q can be answered using a set V of views. We first define the problem in informationtheoretic terms: we say that V determines Q if V provides enough information to uniquely determine the answer to Q. Next, we look at the problem of rewriting Q in terms of V using a specific language. Given a view language V and query language Q, we say that a rewriting language R is complete for VtoQ rewritings if every Q ∈ Q can be rewritten in terms of V∈ V using a query in R, whenever V determines Q. While query rewriting using views has been extensively investigated for some specific languages, the connection to the informationtheoretic notion of determinacy, and the question of completeness of a rewriting language have received little attention. In this article we investigate systematically the notion of determinacy and its connection to rewriting. The results concern decidability of determinacy for various view and query languages, as well as the power required of complete rewriting languages. We consider languages ranging from firstorder to conjunctive queries.
QueryBased Data Pricing
"... Data is increasingly being bought and sold online, and Webbased marketplace services have emerged to facilitate these activities. However, current mechanisms for pricing data are very simple: buyers can choose only from a set of explicit views, each with a specific price. In this paper, we propose a ..."
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Data is increasingly being bought and sold online, and Webbased marketplace services have emerged to facilitate these activities. However, current mechanisms for pricing data are very simple: buyers can choose only from a set of explicit views, each with a specific price. In this paper, we propose a framework for pricing data on the Internet that, given the price of a few views, allows the price of any query to be derived automatically. We call this capability “querybased pricing. ” We first identify two important properties that the pricing function must satisfy, called arbitragefree and discountfree. Then, we prove that there exists a unique function that satisfies these properties and extends the seller’s explicit prices to all queries. When both the views and the query are Unions of Conjunctive Queries, the complexity of computing the price is high. To ensure tractability, we restrict the explicit prices to be defined only on selection views (which is the common practice today). We give an algorithm with polynomial time data complexity for computing the price of any chain query by reducing the problem to network flow. Furthermore, we completely characterize the class of Conjunctive Queries without selfjoins that have PTIME data complexity (this class is slightly larger than chain queries), and prove that pricing all other queries is NPcomplete, thus establishing a dichotomy on the complexity of the pricing problem when all views are selection queries.
Query Answering using Views in the Presence of Dependencies ABSTRACT
"... In this paper we investigate equivalent and maximally contained rewritings of a query using views under a set of dependencies. We propose two new algorithms that find equivalent and maximally contained rewritings in the presence of dependencies, and we also prove that a maximally contained rewriting ..."
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In this paper we investigate equivalent and maximally contained rewritings of a query using views under a set of dependencies. We propose two new algorithms that find equivalent and maximally contained rewritings in the presence of dependencies, and we also prove that a maximally contained rewriting computes all the certain answers under the open world assumption, both in the presence and the absence of dependencies. 1.
Conjunctive Queries Determinacy and Rewriting
"... The problem of whether a query Q can be answered using a set of views V studies the possibility of computing Q when only the answers to the given set of views are available. In informationtheoretic terms, we say that V determines Q iff for any two databases D1, D2, V(D1) = V(D2) implies Q(D1) = Q ..."
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The problem of whether a query Q can be answered using a set of views V studies the possibility of computing Q when only the answers to the given set of views are available. In informationtheoretic terms, we say that V determines Q iff for any two databases D1, D2, V(D1) = V(D2) implies Q(D1) = Q(D2). In the case that V determines Q, we also study the existence of equivalent rewritings of Q in terms of V in a specific rewriting language. Having a view language V and a query language Q we say that VtoQ determinacy is decidable if there is an algorithm which, given a view V ∈ V and a query Q ∈ Q, outputs whether V determines Q. We focus on the case where the views and the query are defined by subclasses of conjunctive queries and investigate in which cases V determines Q and the existence of equivalent rewritings of Q in terms of V. We define the class of CQcgraph queries as binary CQ queries whose body, if viewed as an undirected graph, is connected. Next, we establish necessary conditions for determinacy in the CQcgraphtoCQcgraph case. We also show that CQchaintoCQcgraph determinacy is decidable, extending the previous decidability result for CQchaintoCQchain, where CQchain denotes the class of binary CQ queries whose body is a simple path between the two free variables. Finally, we provide an algorithm which, starting with a set of CQcgraph views V and an integer k, generates a set of CQcgraph queries that are determined by V and have their size bounded by k.
Contents lists available at ScienceDirect Information Systems
"... journal homepage: www.elsevier.com/locate/infosys ..."
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