db/conf/cikm/LiuU97.html Chengwen Liu Andrei Ursu A Framework for Global Optimization of Aggregate Queries. 262-269 CIKM1997/P262.pdf 1997 conf/cikm/97 CIKM http://doi.acm.org/10.1145/266714.266908 db/conf/cikm/cikm97.html#LiuU97