db/journals/jacm/AbiteboulV88.html Serge Abiteboul Victor Vianu Equivalence and optimization of relational transactions. 70-120 1988 35 J. ACM 1 http://doi.acm.org/10.1145/42267.42271 db/journals/jacm/jacm35.html#AbiteboulV88 JACM/35/P0070.pdf