The join operation is one of the most investigated research issues in the context of the relational data model. For over two decades, numerous papers have been published on join-related topics. Alone the overview paper by Mishra and Eich [Mishra and Eich, 1992] refers to 198 join-related publications and there is a huge number of papers that were not mentioned and that have been published since. Join processing has been studied from many different points of view, such as query optimisation, I/O optimisation, buffer usage optimisation, hardware support, parallel processing or physical database design. There are two major reasons why the join has attracted that much attention: