1. An XML database management system (XDBMS) for an XML database comprising XML documents, the XDBMS including at least one computer, each XML document comprising one or more structural elements and adhering to an XML schema, wherein at least one of the structural elements is protected against access of a user, the XDBMS comprising: an optimizer configured to process an XQuery of the user comprising one or more XQuery expressions and further configured to generate an optimized XQuery execution plan; an execution engine configured to execute the optimized XQuery execution plan to retrieve XML data from the XML database, wherein the optimizer is configured to generate the optimized XQuery execution plan, so that all XQuery expressions relating to one or more of the structural elements that are protected against access of the user are ignored by the optimizer.
7. The XDBMS of claim 1 , wherein the execution engine is further configured to evaluate one or more instance-based access privileges attached to at least one of the XML documents, the instance-based access privileges protecting access for the user and/or the groups to the respective XML documents.