Pathfinder: XQuery---the relational way
Peter A. Boncz, Torsten Grust, Maurice van Keulen, Stefan Manegold, Jan Rittinger, Jens Teubner · Data Archiving and Networked Services (DANS) · 2005
Relational query processors are probably the best understood (as well as the best engineered) query engines available today. Although carefully tuned to process instances of the relational model (tables of tuples), these processors can also provide a foundation for the evaluation of "alien" (non-relational) query languages: if a relational encoding of the alien data model and its associated query language is given, the RDBMS may act like a special-purpose processor for the new language.