  External query execution cost modeling using query execution feedback has
found its way in various database applications such as admission control and
query scheduling. Existing techniques in general fall into two categories,
plan-level cost modeling and operator-level cost modeling. It has been shown in
the literature that operator-level cost modeling can often significantly
outperform plan-level cost modeling. In this paper, we study operator-level
cost modeling from a robustness perspective. We address two main challenges in
practice regarding limited execution feedback (for certain operators) and mixed
cost estimates due to the use of multiple cost modeling techniques. We propose
a framework that deals with these issues and present a comprehensive analysis
of this framework. We further provide a case study to demonstrate the efficacy
of our framework in the context of index tuning, which is itself a new
application of external cost modeling techniques.
