1. A data stream management system that processes a plurality of streams of data using a plurality of continuous queries, the data stream management system comprising: a store comprising a plurality of tuples representing a relation, each tuple being time stamped; a memory comprising a global plan currently being used in execution of the continuous queries on the plurality of streams; a processor, coupled to said store, to modify said global plan in said memory in response to receipt of a new continuous query that uses a new aggregation defined by a command, by using an operator to invoke a first instance in a plurality of instances of the new aggregation based on a first value of an attribute of said data; wherein said new aggregation groups data in said streams by said attribute; wherein the processor is configured to execute an instruction in the operator, to perform the new aggregation; wherein a first count corresponds to said first value of said attribute and said first count is changed for data of said first value by the processor; wherein the processor is configured to create the first instance when the first count is changed from an initial value; and wherein the processor is configured to delete the first instance when the first count is changed to return to said initial value.