10. A computer program product for optimizing query evaluation and processing services in a subscription notification system, the computer program product including a computer-readable program of instructions on a non-transitory computer-readable storage medium for implementing a method, comprising: receiving a set of subscription queries at a notification broker system from subscription client systems, the notification broker system comprising processing circuitry and acting as an intermediary between the subscription client systems and a notification service provider system; determining hierarchical relationships among the set of subscription queries received from subscription client systems at the notification broker system, and generating a classification tree by analyzing the subscription queries to identify subscription queries that are subsets of corresponding other subscription queries; selecting one of the nodes in the classification tree as the basis for an optimal subscription query, the selection based on a predefined policy that describes at least one of an ability of the notification service provider system to service subscriptions, an ability of the notification broker system to service subscriptions, and a cost of sending a notification from the notification service provider system to the notification broker system; generating, from a corresponding subscription query identified by the node, at least one optimal subscription query to include all query elements from the selected node and all of the query elements from child nodes of the selected node; subscribing the notification broker system to the notification service provider system with at least one optimal subscription query; receiving at least one notification message from the notification service provider system in response to the at least one optimal subscription query from the notification broker system; and identifying subscription client systems for transmitting the at least one notification message resulting from execution of the at least one optimal subscription query by searching a classification tree for the one or more selected nodes that match the at least one optimal subscription query for which the at least one notification message is generated, and transmitting the at least one notification message to subscriber client systems associated with the one or more selected nodes.
17. The computer program product of claim 10 , wherein the selecting one of the nodes in the classification tree selects the one of the nodes based upon a minimum number of relevant messages expected to be obtained via execution of the selection.