1. An apparatus for processing a data stream, the apparatus comprising: a cluster generator configured to generate at least one cluster of queries based on a commonality of words included in the queries, and to extract a representative query for each generated cluster; a query classification processor configured to, in response to receiving a query, determine a cluster to which the received query belongs, and map the received query to a representative query of the determined cluster; and a query execution unit configured to execute the representative query from the determined cluster to determine an answer of the representative query based on the data stream, and to provide the answer of the representative query as an approximate answer of the received query instead of processing the received query, wherein the queries comprise a feature vector, and the cluster generator is further configured to generate the cluster based on the feature vector of the queries.