On extending the XML engine with query-processing capabilities

We study how to efficiently evaluate queries over XML documents whose representation is according to the XML specification, i.e., XML files. The software architecture is as follows: the XML engine (i.e., XML parser) makes the structure of the documents explicit. The query processor operates directly on the output of the XML engine. We see two basic alternatives of how such a query processor operates: event-based and tree-based. In the first case, the query processor immediately checks for each event, e.g., begin of an element, if it contributes to a query result or if it invalidates current partial results. In the second case, the query processor generates an explicit transient representation of the document structure and evaluates the query set-at-a-time. This work evaluates these approaches and some optimizations in quantitative terms. Our main results are as follows. The event-based evaluation scheme is approximately 10% faster, even with all the optimizations from this article. The overhead of the query processors is small, compared to the running times of the XML engine. Finally exploiting DTD information in this particular context does not lead to a better performance.