1. A method for use in a data processing system having a plurality of processing stages, comprising: associating a membership structure with at least an output structure of at least a particular one of the processing stages, wherein the processing stages perform respective different operations on data received by the data processing system, and wherein the output structure contains partially processed data produced by the particular processing stage; and in response to a query, accessing the membership structure to determine whether the membership structure indicates that data responsive to the query is present in the output structure that contains partially processed data.