1. A high speed data stream monitoring system for monitoring and ascertaining desired characteristics of a high speed data stream flowing in a network, the data in the data stream being in the form of serial tuples, the monitoring system comprising: means for evaluating new tuples arriving in the stream, the means for evaluating including computation means for running a query plan on new tuples arriving in the stream, the query plan including a set of high level queries and a set of different low level queries, the low level queries being characterized by different sets of predicates to be evaluated on the tuples as part of the queries, the set of different low level queries including a plurality of different low level queries sharing a common predicate, a predicate prefilter outside of the query plan that includes a set of predicates selected from the low level query predicates including a common predicate shared by different low level queries and that evaluates the selected predicates on a new tuple arriving in the stream before running any of the low level queries on the tuple and produces a predicate signature for each new tuple in response to the evaluation; means for assigning a predicate signature to each of the low level queries, means for determining those low level queries which have a predicate signature compatible with the predicate signature of the tuple produced by the prefilter; means for applying to the tuple only those low level queries determined to have a predicate signature compatible with the predicate signature of the tuple produced by the prefilter, along with selected high level queries, to ascertain the desired characteristics of the high speed data stream; whereby low level queries that do not have a compatible bit signature are not applied to the tuple by the monitoring system and the computation means has a computation load for running the query plan that is reduced.