11. A computer readable storage medium storing a computer program executable by a processor for indexing documents with respect to a first phrase, the actions of the computer program comprising: storing a primary index of phrases; storing a secondary index of phrases; establishing a list of documents that contain the first phrase; partitioning the documents in the list into at least a first portion comprising higher ranked documents in the list, and a second portion comprising lesser ranked documents in the list, based on ranking the documents in the list by a relevance score; storing the first portion in the primary index, the higher ranked documents of the first portion stored relative to one another in the primary index in rank order of the respective relevance scores of the ranked documents; and based on the partitioning, storing the second portion in the secondary index, the lesser ranked documents of the second portion stored relative to one another in the secondary index in numerical order of the respective document identifiers of the ranked documents, wherein an identifier indicating a reference to the secondary index is stored in the primary index, and is associated with the stored first portion.
13. The computer readable storage medium of claim 11 , wherein storing the second portion of the list in the secondary index comprises storing substantially only document identification information.