XQuery Rewrite Optimization in IBM DB2 pureXML

In this paper, we describe XQuery compilation and rewrite optimization in DB2 pureXML, a hybrid relational and XML database management system. DB2 pureXML has been designed to scale to large collections of XML data. In such a system, effective filtering of XML documents and efficient execution of XML navigation are vital for high throughput. Hence the focus of rewrite optimization is to consolidate navigation constructs as much as possible and to pushdown comparison predicates and navigation constructs into data access to enable index usage. In this paper, we describe the new rewrite transformations we have implemented specifically for XQuery and its navigational constructs. We also briefly discuss how some of the existing rewrite transformations developed for the SQL engine are extended and