1. A computer-implemented method of managing a plurality of streams of data in a computer, the method comprising: processing the plurality of streams, to execute thereon a plurality of continuous queries based on a global plan; during said processing, receiving a new continuous query to be executed; during said processing, identifying from the global plan, a first operator that supplies data on the relation; during said processing, modifying the global plan by coupling the first operator to a second operator to be used to implement the new continuous query, thereby to obtain a modified plan; prior to execution of the new continuous query, propagating from the first operator to the second operator, a current state of a relation if said relation is output by the first operator; altering said processing, to cause execution of the new continuous query in addition to said plurality of continuous queries, based on the modified plan; and outputting from said computer, a stream generated based at least partially on processing of said data by executing the new continuous query.