Relative expressive power of navigational querying on graphs
Proceedings of the 14th International Conference on Database Theory
FoIKS'12 Proceedings of the 7th international conference on Foundations of Information and Knowledge Systems
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We study the expressiveness of a positive fragment of path queries, denoted Path+, on documents that can be represented as node-labeled trees. The expressiveness of Path+ is studied from two angles. First, we establish that Path+ is equivalent in expressive power to two particular subfragments, as well as to the class of tree queries, a subclass of the first-order conjunctive queries defined over the 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 subqueries that can be expressed in a very small fragment 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 a large class of path queries on documents.