Query Rewrite for XML in Oracle XML DB

Abstract

This chapter presents the “XML Query Rewrite” technique used in Oracle XML Database (DB). Oracle XML DB integrates XML storage and querying using the Oracle relational and object relational framework. It has the capability to physically store XML documents by shredding them as relational or object relational data, and creating logical XML documents using SQL/XML publishing functions. However, querying XML in a relational or object relational database poses several challenges. The biggest challenge is to efficiently process queries against XML in a database whose fundamental storage is table based and whose fundamental query engine is tuple oriented. The XML Query Rewrite technique integrates querying XML using XPath embedded inside SQL operators and SQL/XML publishing functions with the object relational and relational algebra. A common set of algebraic rules is used to reduce both XML and object queries into their relational equivalent. This enables a large class of XML queries over XML type tables and views to be transformed into their semantically equivalent relational or object relational queries. These queries are then amenable to classical relational optimizations yielding XML query performance comparable to the relational. Furthermore, this rewrite technique lays out a foundation to enable rewrite of XQuery over XML.