Location privacy receives considerable attentions in emerging location based services. Most current practices however either ignore users' preferences or incompletely fulfill privacy preferences. In this paper, we propose a privacy protection solution to allow users' preferences in the fundamental query of k nearest neighbors (kNN). Particularly, users are permitted to choose privacy preferences by specifying minimum inferred region. Via Hilbert curve based transformation, the additional workload from users' preferences is alleviated. Furthermore, this transformation reduces time-expensive region queries in 2-D space to range the ones in 1-D space. Therefore, the time efficiency, as well as communication efficiency, is greatly improved due to clustering properties of Hilbert curve. Further, details of choosing anchor points are theoretically elaborated. The empirical studies demonstrate that our implementation delivers both flexibility for users' preferences and scalability for time and communication costs.
In the last few years, RDF is becoming the dominating data model used in semantic web for knowledge representation and inference. In this paper, we revisit the problem of pattern matching query in RDF model, which is usually expensive in efficiency due to the huge cost on join operations. To alleviate the efficiency pain, view materialization techniques are usually deployed to accelerate the query processing. However, given an arbitrary view, it remains difficult to identify how to reuse the view for a particular query, because of the NP-hardness behind the algorithm matching patterns and views. To fully exploit the benefit of the materialized views, we propose a new paradigm to enhance the effectiveness of the materialized view. Instead of choosing materialized views in arbitrary form, our paradigm aims to select the views only if they are sortable. The property of sortability raises huge gains on the pattern-view matching, bringing down the cost to linear complexity in terms of the pattern size. On the other side, the costs on identifying sortable views and searching over the views using inverted index are affordable. Moreover, sortable views generally improve the overall performance of pattern matching, by means of a cost model used to optimize the query rewriting on the most appropriate views. Finally, we demonstrate extensive experimental results to verify the superiority of our proposal on both efficiency and effectiveness.
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