Scopus-Elsevier Proceedings of the Eighteenth ACM SIGACT-SIGMOD-SIGART Symposium on Principles of Database Systems (PODS'99, Philidephia PA, USA, May 31-June 2, 1999), 155-166 STARTPAGE=155;ENDPAGE=166;TITLE=Proceedings of the Eighteenth ACM SIGACT-SIGMOD-SIGART Symposium on Principles of Database Systems (PODS'99, Philidephia PA, USA, May 31-June 2, 1999) PODS