1. A method for use in a data processing system having a plurality of processing stages, comprising: producing an output structure by 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; associating a membership structure with the output structure produced by the particular processing stage; and in response to a query received from a query client after the output structure has been produced by the particular processing stage, accessing, by at least one processor, the membership structure to determine whether the membership structure indicates that data responsive to the query is present in the partially processed data in the output structure.