Henk Ernst Blok Sunil Choenni Henk M. Blanken Peter M. G. Apers A Selectivity Model for Fragmented Relations: Applied in Information Retrieval. 635-639 2004 16 IEEE Trans. Knowl. Data Eng. 5 http://csdl.computer.org/comp/trans/tk/2004/05/k0635abs.htm db/journals/tkde/tkde16.html#BlokCBA04