Indian Institute of Science
The automated optimization of declarative user queries is a classical hallmark of database technology. XML, with its support for deep data hierarchies and powerful query operators, including regular expressions and sibling axes, renders the query optimization challenge significantly more complex. In this paper, the authors analyze the behavior of industrial-strength XQuery optimizers using the notion of "Plan diagrams", which had hitherto been applied solely to relational engines. Plan diagrams visually characterize the optimizer's query plan choices on a parametrized query space and extending them to the XML environment requires redesigned definitions of the parameters and the space.