On the performance of the position() XPath function

In very large XML documents or collections, the query response times are not always satisfactory. To overcome this limitation, parallel processing can be applied. Data can be replicated in several processors and queries can be partitioned to run over different virtual data partitions on each processor, on an approach called virtual partitioning. PartiX-VP is a simple XML virtual partitioning approach that generates virtual data partitions by dividing the cardinality of the partitioning attribute by the number of allocated processors, resulting in intervals of equal size for each processor. In this approach, the XML query is rewritten and selection predicates are added to define the virtual partitions. These selection predicates use the position() XPath function that addresses a set of elements on a given position in the document. In this paper, we present an experimental evaluation of the position() XPath function in five XML native DBMS. We have identified differences in the processing time of the position() XPath function in large collections of XML documents. This may lead to load unbalancing in simple virtual partitioning approaches, thus this analysis opens space for improvements in virtual partitioning.