Fragkiskos Pentaris Yannis E. Ioannidis Distributed Query Optimization by Query Trading. 532-550 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2992&spage=532 2004 conf/edbt/2004 EDBT db/conf/edbt/edbt2004.html#PentarisI04