Query Translation from XPath to SQL in the Presence of Recursive DTDs

The interaction between recursion in XPATH and recursion in DTDs makes it challenging to answer XPATH queries on XML data that is stored in an RDBMS via schema-based shredding. We present a new approach to translating XPATH queries into SQL queries with a simple least fixpoint (LFP) operator, which is already supported by most commercial RDBMS. The approach is based on our
algorithm for rewriting XPATH queries into regular XPATH expressions, which are capable of capturing both DTD recursion and XPATH queries in a uniform framework. Furthermore, we provide an algorithm for translating regular XPATH queries to SQL queries with LFP, and optimization techniques for minimizing the use of the LFP operator. The novelty of our approach consists in its capability
to answer a large class of XPATH queries by means of only low-end RDBMS features already available in most RDBMS. Our experimental results verify the effectiveness of our techniques.