1. A computer-implemented method for processing a federated query in a federated database system, the method comprising: receiving a federated query at a federated database server; generating a federated query execution plan, based on the received federated query, the federated query execution plan defining one or more source servers of the federated database and a unique subquery to be executed on each of the defined one or more source servers; distributing the subqueries to the one or more source servers in accordance with the federated query execution plan; executing the respective subqueries asynchronously at the one or more source servers; passing the subquery results to a first designated source server defined in the federated query execution plan; joining and aggregating the subquery results at the first designated source server into a final query result; and returning the final query result to the federated database server.