12. The computer-readable medium of claim 11 , wherein the operations further comprise, for at least a phrase of the identified phrases: selectively partitioning the posting list for the phrase into at least a first portion including identifiers of higher ranked documents and a second portion including identifiers of lesser ranked documents, the partitioning being based on a relevance score for a document identified in the posting list indicating the document's relevance to the phrase; storing the first portion in a primary index on the assigned index server; and storing the second portion in a secondary index on the assigned index server.
16. The computer-readable medium of claim 12 , wherein the first portion has a maximum quantity of document identifiers and the second portion has a remainder of the document identifiers.