Sophie Cluet Guido Moerkotte On the Complexity of Generating Optimal Left-Deep Processing Trees with Cross Products. 54-67 ICDT1995/P054.pdf 1995 ICDT db/conf/icdt/icdt95.html#CluetM95 conf/icdt/95 http://dx.doi.org/10.1007/3-540-58907-4_6 books/fm/GareyJ79 journals/tods/IbarakiK84 conf/vldb/KrishnamurthyBZ86 ... conf/vldb/OnoL90 conf/sigmod/SelingerACLP79