13. A method according to claim 12 , wherein the documents are organized into sets in the storage sub-system, and wherein: the content index is a single content index including index entries for content parts of all the documents, and the metadata index includes a plurality of metadata indexes each including index entries for metadata parts of only a corresponding set of the documents; and the linking structure includes a single content-to-metadata linking structure containing the content-to-metadata entries for all the documents, and a plurality of metadata-to-content linking structures each containing the metadata-to-content links for only a corresponding set of the documents.
13. A method according to claim 12 , wherein the documents are organized into sets in the storage sub-system, and wherein: the content index is a single content index including index entries for content parts of all the documents, and the metadata index includes a plurality of metadata indexes each including index entries for metadata parts of only a corresponding set of the documents; and the linking structure includes a single content-to-metadata linking structure containing the content-to-metadata entries for all the documents, and a plurality of metadata-to-content linking structures each containing the metadata-to-content links for only a corresponding set of the documents. 14. A method according to claim 13 , wherein the sets of documents are selected from local email files, share point files, file system files, hosted email files, hosted document management systems, databases, enterprise resource planning or customer relationship management systems, and instant messaging content.