Yunjun Gao Jing Zhang Gencai Chen Qing Li Shen Liu Chun Chen Optimal-Nearest-Neighbor Queries. 1454-1456 2008 ICDE http://doi.ieeecomputersociety.org/10.1109/ICDE.2008.4497587 conf/icde/2008 db/conf/icde/icde2008.html#GaoZCLLC08