XML views as integrity constraints and their use in query translation

The SQL queries produced in XML-to-SQL query translation are often unnecessarily complex, even for simple input XML queries. In this paper we argue that relational systems can do a better job of XML-to-SQL query translation with the addition of a simple new constraint, which we term the "lossless from XML" constraint. Intuitively, this constraint states that a given relational data set resulted from the shredding of an XML document that conformed to a given schema. We illustrate the power of this approach by giving an algorithm that exploits the "lossless from XML" constraint to translate path expression queries into efficient SQL, even in the presence of recursive XML schemas. We argue that this approach is likely to be simpler and more effective than the current state of the art in optimizing XML-to-SQL query translation, which involves identifying and declaring multiple complex relational constraints and then reasoning about relational query containment in the presence of these constraints.