1. An XML data base management system (XDBMS, 10) for an XML database ( 20 ) comprising XML documents ( 30 ), each XML document ( 30 ) comprising one or more structural elements ( 35 ) and adhering to an XML schema ( 40 ), wherein at least one of the structural elements ( 35 ) is protected against access of a user ( 60 ), the XDBMS ( 10 ) comprising: a. an optimizer ( 300 ) adapted to process an XQuery ( 50 ) of the user ( 60 ) comprising one or more XQuery expressions ( 55 ) and further adapted to generate an optimized XQuery execution plan ( 70 ); b. an execution engine ( 400 ) adapted to execute the optimized XQuery execution plan ( 70 ) to retrieve XML data ( 80 ) from the XML database ( 20 ), characterized in that c. the optimizer ( 300 ) is adapted to generate the optimized XQuery execution plan ( 70 ), so that all XQuery expressions ( 55 ) relating to one or more of the structural elements ( 35 ) which are protected against access of the user ( 60 ) are ignored by the optimizer ( 300 ).