1. A phrase-based indexing system comprising: a first tier of index servers including N index servers, each of which stores a portion of a phrase posting list for each of a plurality of phrases, each phrase posting list being associated with a phrase and a list of documents having at least one occurrence of the phrase; and M additional tiers of index servers, wherein: M is one to a predetermined number, each M th tier includes T index servers, where T is an integer multiple of N when M equals one and where T is an integer multiple of T for an (M−1) th tier when M is greater than or equal to two, and each M th tier index server stores a portion of a phrase posting list of each of a plurality of phrases, each phrase posting list being associated with a phrase and a list of documents having at least one occurrence of the phrase.