Cost-based optimization in DB2 XML

DB2 XML is a hybrid database system that combines the relational capabilities of DB2 Universal Database (TM) (UDB) with comprehensive native XML support. DB2 XML augments DB2 (R) UDB with a native XML store, XML indexes, and query processing capabilities for both XQuery and SQL/XML that are integrated with those of SQL. This paper presents the extensions made to the DB2 UDB compiler, and especially its cost-based query optimizer, to support XQuery and SQL/XML queries, using much of the same infrastructure developed for relational data queried by SQL. It describes the challenges to the relational infrastructure that supporting XQuery and SQL/XML poses and provides the rationale for the extensions that were made to the three main parts of the optimizer: the plan operators, the cardinality and cost model, and statistics collection.