如何为ORDER BY创建索引？

Question

这是我的业务报告，我只需要为ORDER BY创建一个索引。 但是，group by是基于来自不同表的金额和成本的SUM，那么我该如何创建索引呢？

     SELECT 
     TECHNICAL.TEMPLOYEE_ID,CONSULTANT.EMP_NAME, 
     SUM(SERVICES_PERFORMED.SP_AMOUNT * SERVICE.SER_COST) AS TotalCost

     FROM SERVICES_PERFORMED,TECHNICAL,CONSULTANT,SERVICE

     WHERE TECHNICAL.TEMPLOYEE_ID = SERVICES_PERFORMED.TEMPLOYEE_ID AND
     TECHNICAL.TEMPLOYEE_ID = CONSULTANT.EMPLOYEE_ID AND
     SERVICES_PERFORMED.SERVICE_ID = SERVICE.SERVICE_ID

     GROUP BY TECHNICAL.TEMPLOYEE_ID,CONSULTANT.EMP_NAME
     ORDER BY TotalCost DESC;

这是我想到的查询：

CREATE INDEX TCINDX ON  SERVICES_PERFORMED,SERVICE(TotalCost);

但这是不正确的。