Relational Databases for Querying XML Documents: Limitations and Opportunities

TLDR

XML is rapidly becoming the dominant standard for representing data on the World Wide Web. This study investigates using traditional relational database engines to process XML documents conforming to DTDs, identifies limitations of this approach, and proposes extensions to the relational model to improve XML query support. The authors developed algorithms and a prototype system that convert XML documents to relational tuples, translate semi‑structured XML queries into SQL, and convert results back to XML, and qualitatively evaluated this approach on several real DTDs from diverse domains. The relational approach handles most, but not all, semantics of semi‑structured XML queries, and is likely effective only in some cases.

Abstract

XML is fast emerging as the dominant standard for representing data in the World Wide Web. Sophisticated query engines that allow users to effectively tap the data stored in XML documents will be crucial to exploiting the full power of XML. While there has been a great deal of activity recently proposing new semistructured data models and query languages for this purpose, this paper explores the more conservative approach of using traditional relational database engines for processing XML documents conforming to Document Type Descriptors (DTDs). To this end, we have developed algorithms and implemented a prototype system that converts XML documents to relational tuples, translates semi-structured queries over XML documents to SQL queries over tables, and converts the results to XML. We have qualitatively evaluated this approach using several real DTDs drawn from diverse domains. It turns out that the relational approach can handle most (but not all) of the semantics of semi-structured queries over XML data, but is likely to be effective only in some cases. We identify the causes for these limitations and propose certain extensions to the relational model that would make it more appropriate for processing queries over XML documents.