... System¹

This research is supported by a strategic grant from the Natural Science and Engineering Research Council (NSERC) of Canada. The work was conducted while the authors were affiliated to University of Alberta

... Oria²

College of Computing Sciences, New Jersey Institute of Technology,University Heights, Newark, NJ, 07102-1982, oria@cis.njit.edu.

...Özsu³

Department of Computer Science, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, tozsu@db.uwaterloo.ca.

... Iglinski⁴

Department of Computer Science, University of Alberta, Edmonton, Alberta, Canada T6G 2H1, iglinski@cs.ualberta.ca.

... Lin⁵

IBM Canada, Toronto Lab,shulin@ca.ibm.com.

... Yao⁶

Department of Computer Science, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, bbyao@uwaterloo.ca.