1. A continuous query result estimator for use in a data stream management system to reduce data stream processing times, the data stream management system being configured to execute a continuous query against data items received via at least one input data stream to generate at least one output data stream, the continuous query result estimator comprising: a classification module configured to classify a set of at least one data item received via the at least one input data stream into one of a plurality of input data groups; a data store configured to store, in association with each of the input data groups, a respective rule for estimating a result of executing the continuous query against a data item belonging to the input data group; and a result estimation module configured to: select a rule from the rules stored in the data store on the basis of the classification performed by the classification module; and apply the selected rule to the received set of data items to generate an estimate for a result of executing the continuous query against the received set of data items.