Optimizing View Queries in ROLEX to Support Navigable Result Trees

An increasing number of applications use XML data published from relational databases. For speed and convenience, such applications routinely cache this XML data locally and access it through standard navigational interfaces such as DOM, sacrificing the consistency and integrity guarantees provided by a DBMS for speed. The ROLEX system is being built to extend the capabilities of relational database systems to deliver fast, consistent and navigable XML views of relational data to an application via a virtual DOM interface. This interface translates navigation operations on a DOM tree into execution-plan actions, allowing a spectrum of possibilities for lazy materialization. The ROLEX query optimizer uses a characterization of the navigation behavior of an application, and optimizes view queries to minimize the expected cost of that navigation. This paper presents the architecture of ROLEX, including its model of query execution and the query optimizer. We demonstrate with a performance study the advantages of the ROLEX approach and the importance of optimizing query execution for navigation.

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