A study of a positive fragment of path queries: expressiveness, normal form, and minimization
Faculty of Sciences. Mathematics and Computer Science
Berlin :Springer, 2009
Dataspace, the final frontier: proceedings of the 26th British National Conference on Databases, BNCOD 26, Birmingham, UK, July 7-9, 2009
University of Antwerp
We study the expressiveness of a positive fragment of path queries, denoted Path$\mathstrut+$, on node-labeled trees documents. The expressiveness of Path$\mathstrut+$ is studied from two angles. First, we establish that Path$\mathstrut+$ 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$\mathstrut+$ yields a normal form for Path$\mathstrut+$ queries. Using this normal form, we can decompose a Path$\mathstrut+$ query into sub-queries that can be expressed in a very small sub-fragment of Path$\mathstrut+$ for which efficient evaluation strategies are available. Second, we characterize the expressiveness of Path$\mathstrut+$ 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 a large class of path queries on documents.