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IBM Journal of Research and Development  
Volume 45, Number 2, Page 299 (2006)
Celebrating 10 Years of XML
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Cost-based optimization in DB2 XML

by A. Balmin, T. Eliaz, J. Hornibrook, L. Lim, G.M. Lohman, D. Simmen, M. Wang, C. Zhang
DB2 XML is a hybrid database system that combines the relational capabilities of DB2 Universal Database™ (UDB) with comprehensive native XML support. DB2 XML augments DB2® 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.
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