2. The method of claim 1 , wherein determining one of the plurality of query performance engines includes checking whether a query performance engine is predetermined by a user based on the received criterion to select a query performance engine.
2. The method of claim 1 , wherein determining one of the plurality of query performance engines includes checking whether a query performance engine is predetermined by a user based on the received criterion to select a query performance engine. 3. The method of claim 2 , wherein determining one of the plurality of query performance engines includes checking a load on at least one query performance engine to change the determined query performance engine to another query performance engine when the load exceeds a prescribed level.