How big must complete XML query languages be?

Marx and de Rijke have shown that the navigational core of the w3c XML query language XPath is not first-order complete ? that is it cannot express every query definable in firstorder logic over the navigational predicates. How can one extend XPath to get a first-order complete language? Marx has shown that Conditional XPath ? an extension of XPath with an "Until" operator ? is first order complete. The completeness argument makes essential use of the presence of upward axes in Conditional XPath. We examine whether it is possible to get "forward-only" languages that are first-order complete for XML Boolean queries. It is easy to see that a variant of the temporal logic CTL is first-order complete; the variant has path quantifiers for downward, leftward and rightward paths, while along a path one can check arbitrary formulas of linear temporal logic (LTL). This language has two major disadvantages: it requires path quantification in both horizontal directions (in pa...

• Type: Conference
• Year: 2009
• Where: ICDT
• Authors: Clemens Ley, Michael Benedikt
• Keywords: Conditional XPath, Database, First-order Complete Language, ICDT 2009, Query Language Xpath