In today's world everyone uses information technology in the form of social network, business and this information is nothing but a database. Maintaining the largest databases with high performance is called as a database query optimization. A distributed database is a group of autonomous cooperating centralized databases, in that query processing requires transferring data from one system to another through a communication network. In the query optimization process, cost is always associated with each and every Query Execution Plan (QEP). Execution according to plan, cost is the sum of commutation cost, secondary storage access cost, storage cost, computation cost.