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.
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. 3. The physical, non-volatile medium of claim 1 , wherein the constraint type determines what number and type of values are attributed to each constraint of the one or more constraints, and wherein the string type constraint comprises an alphanumeric string, the binary type constraint comprises a yes/no value, the scalar type constraint comprises a ranked value, and the ontology type constraint comprises a hierarchy of classes.