12. A computer-implemented method for providing search results, the method comprising: receiving a search query comprising one or more terms; analyzing the search query to identify one or more atoms from the one or more terms; querying a search index using the one or more atoms identified from the search query, wherein the search index comprises a plurality of postings lists, each posting list corresponding with an atom and including a plurality of postings, wherein each posting within a given posting list corresponding with an atom identifies a document containing the atom and a rank representing a significance of the atom for the document, wherein at least one posting list is divided into a plurality of tiers ordered based on rank, and wherein querying the search index comprises determining whether to perform additional processing, to identify a plurality of documents, based on comparing a combined rank of a first set of documents identified after a first tier with a calculated combined rank of the first set of documents of the first tier and one or more documents of at least a second tier; identifying a plurality of documents from querying the search index based on the plurality of tiers ordered based on rank and the postings within each of the plurality of tiers; and providing a plurality of search results for presentation to an end user based on the plurality of documents identified by querying the search index.
13. The computer-implemented method of claim 12 , wherein a number of tiers for at least one positing list and a number of documents within each tier of the at least one positing list is determined based on the size of the document corpus used to generate the search index.