15. The method of claim 14 , wherein the set of document identification tags for a first segment includes a plurality of document identification tags in the set of document identification tags for a second segment.
14. A computer-implemented method of assigning a plurality of document identification tags to a plurality of new documents, the plurality of new documents to be added to a collection of documents, the method comprising: partitioning a set of valid globally unique document identifiers into a plurality of segments, each segment associated with a respective subset of the set of valid globally unique document identifiers; subdividing each of the segments into a plurality of tiers, wherein the plurality of tiers are monotonically ordered with respect to a query-independent document importance metric, each segment having an associated, predetermined set of monotonically ordered document identification tags, and each tier of a respective segment having an associated subset of the set of document identification tags for the respective segment; receiving query-independent information about a new document, the information including the query-independent document importance metric and a globally unique document identifier; selecting, based at least in part on the globally unique document identifier, one of the segments; selecting, based at least on the query-independent information, one of the tiers associated with the selected segment; assigning to the new document a document identification tag from the subset of document identification tags associated with the selected tier, wherein the document identification tag assigned to the new document is unique with respect to document identification tags assigned to other documents in the collection of documents; storing an assignment of the document identification tag from the subset of document identification tags associated with the selected tier to the new document in a computer-readable medium; and repeating the receiving, selecting a segment, selecting a tier, assigning, and storing with respect to one or more additional new documents; wherein the assigned document identification tags are assigned to documents in the collection of documents having globally unique document identifiers associated with the respective segment.