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; prior to scheduling an operator of the one or more operators, determining when eviction is to be performed based a level of free memory of the processing system; determining an eviction candidate; and evicting, from the memory to a persistent storage, a page associated with the eviction candidate.