Issue No.09 - September (1983 vol.32)
W.W. Chu , Department of Computer Science, School of Engineering and Applied Science, University of California
In the above paper1, Theorem 1 is not true in general. The purpose of Theorem 1 was to reduce the number of feasible query trees by determining the optimal positions of unary operations in a given query tree. In fact, Theorem I is true only if the processing cost of a unary operation is independent of the volume of input data.
W.W. Chu, P. Hurley, "Correction to "Optimal Query Processing for Distributed Database Systems"", IEEE Transactions on Computers, vol.32, no. 9, pp. 878, September 1983, doi:10.1109/TC.1983.1676340