29. A computer program product for use in conjunction with a computer system, the computer program product comprising a computer readable storage medium and a computer program mechanism therein, the computer program mechanism comprising: a data structure representing a subdivision of a predetermined set of monotonically ordered document identification tags into a plurality of tiers, wherein the plurality of tiers are monotonically ordered with respect to a query-independent document importance metric, wherein each tier is associated with a respective subset of the set of document identification tags, and wherein the data structure includes a representation of a monotonic ordering of the tiers; an interface configured to receive query-independent information about a new document, the information including the query-independent document importance metric; and a tag assignment module including instructions for selecting, based at least on the query-independent information, one of the tiers; and assigning to the new document a document identification tag from the respective subset of document identification tags associated with the selected tier, the assigned document identification tag not previously assigned to any of the documents in a collection of documents.
29. A computer program product for use in conjunction with a computer system, the computer program product comprising a computer readable storage medium and a computer program mechanism therein, the computer program mechanism comprising: a data structure representing a subdivision of a predetermined set of monotonically ordered document identification tags into a plurality of tiers, wherein the plurality of tiers are monotonically ordered with respect to a query-independent document importance metric, wherein each tier is associated with a respective subset of the set of document identification tags, and wherein the data structure includes a representation of a monotonic ordering of the tiers; an interface configured to receive query-independent information about a new document, the information including the query-independent document importance metric; and a tag assignment module including instructions for selecting, based at least on the query-independent information, one of the tiers; and assigning to the new document a document identification tag from the respective subset of document identification tags associated with the selected tier, the assigned document identification tag not previously assigned to any of the documents in a collection of documents. 39. The computer program product of claim 29 , wherein the tag assignment module further includes instructions for detecting satisfaction of a flush condition and for, when the flush condition is satisfied, performing a flush operation, the flush operation including building a first sorted map and a second sorted map; wherein the first sorted map is keyed and sorted by globally unique identifiers, and includes for each globally unique identifier a corresponding document identification tag; and wherein the second sorted map is keyed and sorted by document identification tags assigned to documents since a prior flush operation, and includes for each such document identification tag a corresponding globally unique identifier.