Pushing XML Queries inside Relational Databases
Résumé
Due to the increasing usage of XML as an exchange format, querying XML documents is also gaining attention. While several languages exist, work is under way to define the W3C standard for an XML query language. To take full advantage of the strength of the RDBMS, XML queries on XML virtual documents need to be translated to SQL queries to be executed by a relational engine. Several existing projects propose mappings between XML documents and relational databases, but the query translation problem from XML queries to SQL queries received less attention. We propose a general method for pushing XML queries into a relational database containing the XML document's data. Our solution is general in that it fits a wide range of mappings between the relational data and the XML documents; it makes a maximal use of the relational engine query capabilities, and relies on well-known algorithms from the relational database field, which makes it a good candidate solution for adding XML query capabilities to a relational system. We also investigate the limits of the translation process in general - not everything can be translated, given the big difference between the expressive power of SQL and that to be expected from XML query languages; we provide an analysis of language features to be expected from the standard XML query language, and establish which of them can be translated to SQL and how.