10. A method of estimating a result of a continuous query in a data stream management system to reduce data stream processing times therein, the data stream management system being operable configured to execute the continuous query against data items received via at least one input data stream to produce at least one output data stream, the method comprising: classifying 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; storing in a data 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 selecting a rule from the rules stored in the data store on the basis of the classification; and applying 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.
10. A method of estimating a result of a continuous query in a data stream management system to reduce data stream processing times therein, the data stream management system being operable configured to execute the continuous query against data items received via at least one input data stream to produce at least one output data stream, the method comprising: classifying 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; storing in a data 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 selecting a rule from the rules stored in the data store on the basis of the classification; and applying 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. 16. The method according to claim 10 , wherein each of the rules comprises a decision tree whose branches can be navigated to arrive at the estimate of the result of executing the continuous query based on values of the received set of data items.