18. A method for providing search results in response to a search query using a staged process comprising: receiving a search query; identifying one or more atoms from the search query; identifying an initial set of documents containing the one or more atoms from a search index, the search index storing a plurality of atoms, the plurality of atoms comprising one or more n-grams, one or more n-tuples, and one or more near n-tuples; computing a preliminary score for each document in the initial set of documents using both a simplified scoring function and pre-computed scores stored in the search index for document/atom pairs for the one or more atoms and the initial set of documents; selecting a subset of documents for further processing based on the preliminary scores; computing a ranking score for each document in the subset of documents using a final ranking algorithm; and providing a set of search results based on the ranking scores.
20. The one or more computer storage media of claim 18 , wherein selecting the subset of documents for further processing based on the preliminary scores comprises selecting a predetermined number of documents having the highest preliminary scores, wherein the predetermined number of documents is based on a fidelity confidence for the simplified scoring function.