1. A method for managing memory usage in a processing system, the method comprising: receiving a data stream comprising a plurality of tuples; determining a query plan generated for a continuous query applied to the plurality of tuples in the data stream, the query plan including one or more operators; determining a level of free memory associated with the processing system; prior to scheduling an operator of the one or more operators, determining whether the operator of the one or more operators can be scheduled based at least in part on the level of free memory of the processing system; in response to determining that the operator of the one or more operators cannot be scheduled, determining an eviction candidate; and evicting, from the memory to a persistent storage, a page associated with the eviction candidate.