Database Systems R&D Center, College of Engineering, University of Florida, USA. yhuang@mail.mcg.edu|Database Systems R&D Center, College of Engineering, University of Florida, USA. yhuang@mail.mcg.edu|Database Systems R&D Center, College of Engineering, University of Florida, USA. yhuang@mail.mcg.edu|Database Systems R&D Center, College of Engineering, University of Florida, USA. yhuang@mail.mcg.edu