Binary Information Press
In order to provide a more powerful XML query ability, an xtwig query, represented as an unrooted labeled tree is proposed. It contains reverse axes in predicates, and specifies the pattern of selection predicates on multiple elements from both descendants and ancestors. A number of algorithms have been proposed recently to process a twig query holistically. Those algorithms, however, only deal with twig queries without reverse axes. A straightforward approach that first decomposes an xtwig query into multiple twig queries and then merges their results is obviously not optimal in most cases.