17. The system of claim 15 , wherein said generating the first pseudo-document similarity score for the individual candidate strings comprises: receiving a set of pseudo-documents, an individual pseudo-document in the set of pseudo-documents corresponding to a respective document in the set of documents, and the individual pseudo-document containing a set of terms found in queries that have been submitted and which have resulted in the selection of the respective document; determining, for the individual candidate strings, a number of pseudo-documents that contain all of the terms of the set in the individual candidate strings; and generating the first pseudo-document similarity score for the individual candidate strings using said number.
17. The system of claim 15 , wherein said generating the first pseudo-document similarity score for the individual candidate strings comprises: receiving a set of pseudo-documents, an individual pseudo-document in the set of pseudo-documents corresponding to a respective document in the set of documents, and the individual pseudo-document containing a set of terms found in queries that have been submitted and which have resulted in the selection of the respective document; determining, for the individual candidate strings, a number of pseudo-documents that contain all of the terms of the set in the individual candidate strings; and generating the first pseudo-document similarity score for the individual candidate strings using said number. 20. The system of claim 17 , wherein said determining the number of pseudo-documents comprises: generating an inverted index that relates pseudo-documents to terms found in the set of candidate strings; generating another inverted index that relates candidate strings to terms found in the set of candidate strings; generating a matrix that identifies, for individual pairs of a particular pseudo-document and a particular candidate string, a number of terms in the particular candidate string that are found in the particular pseudo-document; and determining the number of pseudo-documents using the matrix.