A Study of a Positive Fragment of Path Queries: Expressiveness, Normal Form, and Minimization

We study the expressiveness of a positive fragment of path queries, denoted Path+ , on node-labeled trees documents. The expressiveness of Path+ is studied from two angles. First, we establish that Path+ is equivalent in expressive power to a particular sub-fragment as well as to the class of tree queries, a sub-class of the first-order conjunctive queries defined over label, parent-child, and child-parent predicates. The translation algorithm from tree queries to Path+ yields a normal form for Path+ queries. Using this normal form, we can decompose a Path+ query into sub-queries that can be expressed in a very small subfragment of Path+ for which efficient evaluation strategies are available. Second, we characterize the expressiveness of Path+ in terms of its ability to resolve nodes in a document. This result is used to show that each tree query can be translated to a unique, equivalent, and minimal tree query. The combination of these results yields an effective strategy to evaluate...

• Type: Conference
• Year: 2009
• Where: BNCOD
• Authors: Yuqing Wu, Dirk Van Gucht, Marc Gyssens, Jan Paredaens
• Keywords: BNCOD 2007, BNCOD 2009, Normal Form, Path Queries, Tree Queries