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Abstract: Nowadays, location-related information is highly accessible to mobile users via issuing Location-Dependent Spatial Queries (LDSQs) with respect to their locations wirelessly to Location-Based Service (LBS) servers. Due to the limited mobile device battery energy, scarce wireless bandwidth, and heavy LBS server workload, the number of LDSQs submitted over wireless channels to LBS servers for evaluation should be minimized as appropriate. In this paper, we exploit query containment techniques for LDSQs (called LDSQ containment) to enable mobile clients to determine whether the result of a new LDSQ Q′ is completely covered by that of another LDSQ Q previously answered by a server (denoted by Q′ ⊆ Q) and to answer Q′ locally if Q′ ⊆ Q. Thus, many LDSQs can be reduced from server evaluation. To support LDSQ containment, we propose a notion of containment scope, which represents a spatial area corresponding to an LDSQ result wherein all semantically matched LDSQs are answerable with the result. Through a comprehensive simulation, our proposed approach significantly outperforms existing techniques. [Copyright &y& Elsevier]

Abstract: We present the Constructive Query Containment (CQC) method to check query containment and query containment under constraints for queries over databases with safe negation in both IDB and EDB subgoals and with or without built-in predicates. The aim of the CQC method is to construct a counterexample that proves that the query containment relationship being checked does not hold. The method uses different Variable Instantiation Patterns (VIPs) to generate only relevant counterexamples according to the syntactic properties of the queries and the databases considered in each test. The main contribution of the CQC method is threefold: it handles broader cases of queries and database schemas than most previous methods, it checks “true” containment instead of uniform containment (which is a sufficient but not necessary condition for containment) and it is not less efficient than other methods for the cases that they handle. Moreover, we prove also soundness and completeness of our method both for success and for failure. [Copyright &y& Elsevier]

To improve the query performance over XML documents in a distributed environment, we develop a semantic caching system named ACE-XQ for XQuery queries. ACE-XQ applies innovative query containment and rewriting techniques to answer user queries using cached queries. We also design a fine-grained replacement strategy which records user access statistics at a finer granularity than the complete XML query regions. As a result, less frequently used XML view fragments are replaced to maintain a better utilization of the cache space. Extensive experimental results illustrate the performance improvement achieved by this strategy over the traditional one for a variety of situations. [Copyright &y& Elsevier]