12. 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: on a computer system having one or more processors and memory storing one or more programs for execution by the one or more processors: 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 a value of the query-independent document importance metric and a globally unique document identifier for the new document; 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; and repeating the receiving, selecting a segment, selecting a tier, and assigning, 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.
13. The method of claim 12 , 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.