http://doi.acm.org/10.1145/190627.190631 db/journals/sigmod/Bertram94.html Martin Bertram 0002 8-12 sigmodR/23-4/P008.pdf 1994 23 SIGMOD Record 4 db/journals/sigmod/sigmod23.html#Bertram94 http://www.acm.org/sigmod/record/issues/9412/modeling.txt