With the growing demand for data management, it is a great challenge to reduce and throttle the energy consumption in data centers. DBMS is a vital software in data center to deal with volumes of data, thus, energy efficient query processing and optimization is one of the critical issues to be solved. We proposed a novel cost model to estimate energy consumption of a query plan, and we investigated the effects of different optimization goals on query optimization under different hardware configurations. Experiements demonstrated that the results of performance- oriented and energy-oriented query optimization are same under the traditional hardware configuration, while the results turned out to be different under the new hardware configuration, which indicates that it is practical to improve the energy efficiency of database systems.