Pathfinder is our XQuery to relational compiler developed at the Database Group in cooperation with CWI Amsterdam and TU München. The Pathﬁnder project is an exploration of how far we can push the idea of using mature relational database management technology to design and build a full-ﬂedged XML database. Pathﬁnder requires only local extensions to the underlying DBMS’s kernel, such as the staircase join operator, a join recognition logic in our compiler, as well as a careful consideration of order properties of relational operators. The compiler is part of MonetDB/XQuery, which is claimed to be the world's most efficient XQuery database system.
Read more: http://www.pathfinder-xquery.org