7. A non-transitory computer-readable storage medium storing a computer program executable by at least one processor for indexing documents, actions of the computer program comprising: determining a set of documents related to a first phrase; determining a relevance score for each of the documents in the set, the relevance score for a document indicating the document's relevance to the first phrase; partitioning the set of documents by the relevance scores into at least a first sub-set and a second sub-set, the first sub-set having relevance scores higher than the second sub-set; generating a posting list for the first phrase partitioned across at least two tiers, the posting list comprising document entries that include a document identifier and the relevance score for the document, wherein a first tier stores document entries from the first sub-set and a second tier stores document entries from the second sub-set; repeating the determining, partitioning, and generating for a second phrase; and storing the posting lists in an index.
7. A non-transitory computer-readable storage medium storing a computer program executable by at least one processor for indexing documents, actions of the computer program comprising: determining a set of documents related to a first phrase; determining a relevance score for each of the documents in the set, the relevance score for a document indicating the document's relevance to the first phrase; partitioning the set of documents by the relevance scores into at least a first sub-set and a second sub-set, the first sub-set having relevance scores higher than the second sub-set; generating a posting list for the first phrase partitioned across at least two tiers, the posting list comprising document entries that include a document identifier and the relevance score for the document, wherein a first tier stores document entries from the first sub-set and a second tier stores document entries from the second sub-set; repeating the determining, partitioning, and generating for a second phrase; and storing the posting lists in an index. 9. The computer-readable storage medium of claim 7 , wherein the posting lists of the index are distributed amongst a plurality of computing devices by a hash of a phrase for the posting list.