Raghu Ramakrishnan Divesh Srivastava Semantics and Optimization of Constraint Queries in Databases. 14-17 1994 17 IEEE Data Eng. Bull. 2 DEBU/94JUN-CD.PDF db/journals/debu/debu17.html#RamakrishnanS94 http://sites.computer.org/debull/94JUN-CD.pdf conf/pods/KanellakisKR90 conf/pods/LevyS92 journals/jlp/Ramakrishnan91 journals/jlp/SrivastavaR93 conf/ppcp/SrivastavaRR94