"Contrary to common belief, performance evaluation is an art." (Raj Jain, 1991) Successful performance evaluation may not be achieved with merely executing common benchmarking tools. This talk presents fundamental principles of performance evaluation and how you can put them into practice. Do you understand what exactly "pgbench" does? Is it appropriate workload for your performance evaluation goal? Common benchmarking tools like "pgbench" are handy for just comparing system A and system B, but if you intend to deeply understand the performance of your system, answers to these questions are critical. In order to conduct a meaningful performance evaluation, the methodology should be elegantly designed to meet the goal of the evaluation: choose metrics for the goal, and choose observation techniques for the metrics. Each step requires careful consideration and deep knowledge about the target system. It cannot be done mechanically. This is why performance evaluation is an art. This talk presents principles of designing performance evaluations and shows how you can put them into practice by introducing the speaker's experiences of performance evaluations with PostgreSQL.