232 Automated Physical Database Design and Tuning
are, although important, less relevant. The reason is that using execu-
tion costs potentially introduces additional variables that are outside
the scope of the evaluated tool. When purely evaluating the quality of a
physical design tuner, we should be careful to freeze any external vari-
ables. It is therefore reasonable to assume that the optimizer is correct
and the physical design tool exploits accurate information. Using the op-
timizer’s expected cost rather that the actual execution cost of queries
has precisely that effect, provided that the optimizer is operating under
the same statistical model for all configurations.
It is important to note the we can execute only what the optimizer
considers ...