Efficient Implementation of XQuery Constructor Expressions

Element constructor is one of most expensive operations of the XQuery language as it requires deep copy of the nodes which make up the content of the constructed element. In this paper we propose various optimization and implementation techniques to avoid copying of the nodes during constructor evaluation. The proposed techniques are based on using special kind of XQuery constructors with modified semantics which evaluation does not require content node copying. We also provide optimization rules which replace standard constructors with modified ones without changing query result. The proposed techniques are designed to minimize modifications of an existing implementation. Possible technique extensions which might depend on implementationspecific features are also considered. We present results from experimental study of the techniques which demonstrate performance improvement of constructor evaluation.