Peter M. G. Apers Stefano Ceri Richard T. Snodgrass 177-178 2002 11 VLDB J. 3 http://springerlink.metapress.com/openurl.asp?genre=article&issn=1066-8888&volume=11&issue=3&spage=177 db/journals/vldb/vldb11.html#ApersCS02