Eager Evaluation of Partial Tree-Pattern Queries on XML Streams

Current streaming applications have stringent requirements on query response time and memory consumption because of the large (possibly unbounded) size of data they handle. Further, known query evaluation algorithms on streaming XML documents focus almost exclusively on tree-pattern queries (TPQs). However recently, requirements for flexible querying of XML data have motivated the introduction of query languages that are more general and flexible than TPQs. These languages are not supported by known algorithms. In this paper, we consider a language which generalizes and strictly contains TPQs. Queries in this language can be represented as dags enhanced with constraints. We explore this representation to design an original polynomial time streaming algorithm for these queries. Our algorithm avoids storing and processing matches of the query dag that do not contribute to new solutions (redundant matches). Its key feature is that it applies an eager evaluation strategy to quickly determine when node matches should be returned as solutions to the user and also to proactively detect redundant matches. We experimentally test its time and space performance. The results show the superiority of the eager algorithm compared to the only known algorithm for this class of queries which is a lazy algorithm.