Energy-efficient DBMS has become a hot research topic for recent years. CPU DVFS is an effective dynamic power management scheme. This paper looks into the relationships between performance, energy and power in PostgreSQL under different governors with the aim to discover the power saving effectiveness in DBMS from hardware perspective. After running 22 TPC-H benchmark queries, we summarize the effects of 4 governors on different query processing operators. The experiment results show that CPU DVFS is effective on DBMS energy-efficiency, and different query operators have their respective power usage characteristics and can be exploited to design more efficient customized governors.