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.
P. Hurley, W.W. Chu, "Correction to "Optimal Query Processing for Distributed Database Systems"", IEEE Transactions on Computers, vol. 32, no. , pp. 878, September 1983, doi:10.1109/TC.1983.1676340