1. A computer implemented method of indexing documents of a document collection, wherein each indexed document has a document identifier, the method comprising: storing a posting list of identifiers of documents of the document collection that contain a phrase; partitioning the posting list, by operation of at least one processor, 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; and storing the second portion in a secondary index.