p277-su - Semantic Query Optimization for XQuery over XML...

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Semantic Query Optimization for XQuery over XML Streams Hong Su Elke A. Rundensteiner Murali Mani Worcester Polytechnic Institute 100 Institute Road, Worcester, MA, USA { suhong | rundenst | [email protected] } Abstract We study XML stream-specific schema-based optimization. We assume a widely-adopted automata-based execution model for XQuery evaluation. Criteria are established regard- ing what schema constraints are useful to a particular query. How to apply multiple opti- mization techniques on an XQuery is then ad- dressed. Finally we present how to correctly and efficiently execute a plan enhanced with our SQO techniques. Our experimentation on both real and synthetic data illustrates that these techniques bring significant performance improvement with little overhead. 1 Introduction Using schema knowledge to optimize queries, known as semantic query optimization (SQO), has generated promising results in deductive [21], relational [16] and object databases [12]. Naturally, it is also expected to be an optimization direction for XML stream query processing. Among the three major functionalities of an XML query language, namely, pattern retrieval, fil- tering (e.g., join) and restructuring (e.g., group-by), only pattern retrieval is specific to the XML data model. Therefore, recent work on XML SQO tech- niques [2, 6, 7, 9, 14] focuses on pattern retrieval opti- mization. Most of them fall into one of the following two categories: 1. Techniques in the first category are applicable to both persistent and streaming XML. For exam- ple, query tree minimization [2, 22] would simplify a query asking for “all auctions with an initial price” to Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Very Large Data Base Endowment. To copy otherwise, or to republish, requires a fee and/or special permission from the Endowment. Proceedings of the 31st VLDB Conference, Trondheim, Norway, 2005 one asking for “all auctions”, if it is known from the schema that each auction must have an initial price. The pruned query is typically more efficient to evalu- ate than the original one, regardless of the nature of the data source. 2. Techniques in the second category are only appli- cable to persistent XML. For example, “query rewrit- ing using state extents” [14] assumes that indices are built on element types. In persistent XML applica- tions, it is practical to preprocess the data to build indices. However, this is not the case for the XML stream scenario since data arrives on the fly and usu- ally no indices are provided in the data.
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