XQuery programs can be hard to write and port to new data collections because the path expressions in a query aredependenton the hierarchy of the data. We propose to demonstrate a system to liberate query writers from this dependence. Aplug-and-play querycontains a specification of what data the query needs in order to evaluate. We implementedvirtual eXist-dbto support plug-and-play XQuery queries. Our system adds avirtualDocfunction that lets a programmer sketch the hierarchy needed by the query, which may well be different than what the data has, and logically (not physically) transforms the data (with information loss guarantees) to the hierarchy specified by thevirtualDoc.The demonstration will consist of a sequence of XQuery queries using a virtual hierarchy, including queries suggested by the audience. We will also demonstrate a GUI tool to construct a virtual hierarchy.

Prefix-based numbering is a popular method for numbering nodes in a hierarchy. But prefix-based numbering breaks down when a node's location within a hierarchy changes, such as when XML data is queried after being transformed by an XSLT program or when data is reformatted in the return clause of an inner FLWR expression in a nested XQuery program. A query on transformed data cannot be evaluated as efficiently since the extant prefix-based node numbers cannot be used (unless the data is materialized and then renumbered, which can be expensive). In this paper we present a novel strategy to virtually transform the data without instantiating and renumbering. Our method, which we call virtual prefix-based numbering, couples each prefix-based node number with a level array that locates the node in the numbering space of the virtual hierarchy. The virtual numbering space preserves the property that location-based relationships between nodes can be determined by comparing (virtual) numbers.