92 Automated Physical Database Design and Tuning
ultimate cost of an expression in fact depends on the actual configuration,
and thus any aggressive pruning might result in the loss of optimal plans for
some valid configurations.
5.4.1.1 Pruning the Search Space
A simple solution for this problem would be to simply remove the pruning
altogether. While this approach is correct, it might result in much longer
optimization calls. We can, however, improve this approach by eliminating
candidates that cannot be part of a solution under any configuration. One
of the difficulties of handling APR operators is that they are really a spec-
ification of the required properties that any execution subplan must satisfy.
Therefore, there is no precise notion of the cost ...