1. A physical, non-volatile machine-readable medium including instructions which when executed in a processing system process a query input by a user to a dialog system by: defining a default order of processing constraints identified in a query, each constraint representing a characteristic of an object or action in the query; categorizing each constraint by constraint type, wherein the constraint type is selected from the group consisting of: string, binary, scalar, and ontology constraint types, and wherein each constraint type dictates how a corresponding constraint is modified by a modification strategy; defining an optimum value range specifying an optimum number of results returned in response to the query; applying a selected modification strategy to one or more constraints in accordance with the default order to generate a modified constraint if a number of results returned in response to the query is not within the optimum value range, the selected modification strategy comprising at least one of: relaxing a constraint, removing a constraint, tightening a constraint, and adding a constraint, wherein the selected modification strategy is selected based on factors comprising at least one of: ontological information of the query and prosodic information of the query; processing the query to obtain a number of results for the query; processing a re-query based on the modified constraint to obtain a second number of results; and applying one of a different modification strategy to a same constraint or a second modification strategy to a subsequent constraint in the default order if the number of results for the re-query is not within the defined value range, wherein the step of tightening a constraint comprises: accessing the ontological information, the ontological information comprising a plurality of domain types, each domain type comprising a plurality of subtypes, and a plurality of sub-subtypes arranged in a hierarchical structure, wherein a first constraint identified in the query is associated a first domain type; determining that the first constraint corresponds to a specific subtype from the first domain type; consulting information from a user model to determine whether the re-query should include a first specific sub-subtype or a second specific sub-subtype from the first domain type, the first and second specific sub-subtypes being at a level below the specific subtype; and determining that the re-query should include the first specific sub-subtype because the information from the user model comprises user data indicating a preference for the first specific sub-subtype.