Semantic mappings between data sources play a key role in several data sharing architectures. Mappings provide the relationships between data stored in different sources, and therefore enable answering queries that require data from other nodes in a data sharing network. Composing mappings is one of the core problems that lies at the heart of several optimization methods in data sharing networks, such as caching frequently traversed paths and redundancy analysis.

We consider the problem of answering queries using views, where queries and views are conjunctive queries with arithmetic comparisons (CQACs) over dense orders. Previous work only considered limited variants of this problem, without giving a complete solution. We have developed an ecient algorithm to obtain maximally contained rewritings (MCRs) for queries having left (or right) semi-interval-comparison predicates. For semi-interval queries we show that at least recursive datalog is necessary to nd a maximally-contained solution, and identify cases where datalog is sucient. Finally, we show that obtaining equivalent rewritings for CQAC's is decidable

We study the problem of generating efficient, equivalent rewritings using views to compute the answer to a query. We take the closed-world assumption, in which views are materialized from base relations, rather than views describing sources in terms of abstract predicates, as is common when the open-world assumption is used. In the closedworld model, there can be an infinite number of different rewritings that compute the same answer, yet have quite different performance. Query optimizers take a logical plan (a rewriting of the query) as an input, and generate efficient physical plans to compute the answer. Thus our goal is to generate a small subset of the possible logical plans without missing an optimal physical plan.

As a result of the extensive research in view-based query processing, three notions have been identified as fundamental, namely rewriting, answering, and losslessness. Answering amounts to computing the tuples satisfying the query in all databases consistent with the views. Rewriting consists in first reformulating the query in terms of the views and then evaluating the rewriting over the view extensions. Losslessness holds if we can answer the query by solely relying on the content of the views. While the mutual relationship between these three notions is easy to identify in the case of conjunctive queries, the terrain of notions gets considerably more complicated going beyond such a query class. In this paper, we revisit the notions of answering, rewriting, and losslessness and clarify their relationship in the setting of semistructured databases, and in particular for the basic query class in this setting, i.e., two-way regular path queries. Our first result is a clean explanation of the relationship between answering and rewriting, in which we characterize rewriting as a

In this paper we study the following problem. Given a database and a set of queries, we want to find, in advance, a set of views that can compute the answers to the queries, such that the size of the viewset (i.e., the amount of space, in bytes, required to store the viewset) is minimal on the given database. This problem is important for many applications such as distributed databases, data warehousing, and data integration. We explore the decidability and complexity of the problem for workloads of conjunctive queries. We show that results di#er significantly depending on whether the workload queries have self-joins. If queries can have selfjoins, then a disjunctive viewset can be a better solution than any set of conjunctive views. We show that the problem of finding a minimal-size disjunctive viewset is decidable, and give an upper bound on its complexity. If workload queries cannot have self-joins, there is no need to consider disjunctive viewsets, and we show that the problem is in NP. We describe a very compact search space of conjunctive views, which contains all views in at least one optimal disjunctive viewset. We give a dynamic-programming algorithm for finding minimal-size disjunctive viewsets for queries without self-joins, and discuss heuristics to make the algorithm e#cient.

When a client submits a set of XPath queries to a XML database on a network, the set of answer sets sent back by the database may include redundancy in two ways: some elements may appear in more than one answer set, and some elements in some answer sets may be subelements of other elements in other (or the same) answer sets. Even when a client submits a single query, the answer can be self-redundant because some elements may be subelements of other elements in that answer. Therefore, sending those answers as they are is not optimal with respect to communication costs. In this paper, we propose a method of minimizing communication costs in XPath processing over networks. Given a single or a set of queries, we compute a minimal-size view set that can answer all the original queries. The database sends this view set to the client, and the client produces answers from it. We show algorithms for computing such a minimal view set for given queries. This view set is optimal; it only includes elements that appear in some of the final answers, and each element appears only once.

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Abstract. Answering queries using views is the problem which examines how to derive the answers to a query when we only have the answers to a set of views. Constructing rewritings is a widely studied technique to derive those answers. In this paper we consider the problem of existence of rewritings in the case where the answers to the views uniquely determine the answers to the query. Specifically, we say that a view set V determines a query Q if for any two databases D1, D2 it holds: V(D1) = V(D2) implies Q(D1) = Q(D2). We consider the case where query and views are defined by conjunctive queries and investigate the question: If a view set V determines a query Q, is there an equivalent rewriting of Q using V? We present here interesting cases where there are such rewritings in the language of conjunctive queries. Interestingly, we identify a class of conjunctive queries, CQpath, for which a view set can produce equivalent rewritings for “almost all ” queries which are determined by this view set. We introduce a problem which relates determinacy to query equivalence. We show that there are cases where restricted results can carry over to broader classes of queries. 1

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Data cube construction is a commonly used operation in data warehouses. Because of the volume of data that is stored and analyzed in a data warehouse and the amount of computation involved in data cube construction, it is natural to consider parallel machines for this operation.