Testing Containment of XPath Expressions in Order to Reduce the Data Transfer to Mobile Clients

Within mobile client-server applications which access a server-side XML database, XPath expressions play a central role in querying for XML fragments. Whenever the mobile client can use a locally stored previous query result in order to answer a new query instead of accessing the server-side database, this can significantly reduce the data transfer from the server to the client. In order to check whether or not a previous query result can be reused for a new XPath query, we present a containment test of two XPath queries which combines two steps. At first, we use the DTD in order to check whether all paths selected by one XPath expression are also selected by the other XPath expression. Then we right-shuffle predicate filters of both expressions and start a subsumption test on them.