46. An indexing system for indexing documents in a document collection, the system comprising: a plurality of index servers; one or more memory devices configured store executable instructions; and one or more processors configured to execute the stored instructions to cause the system to: determine a phrase posting list associated with a first phrase, the phrase posting list identifying documents of the document collection associated with the first phrase; divide the phrase posting list for the first phrase into a plurality of shards, each shard identifying a subset of the documents identified by the posting list; store each shard of the phrase posting list for the first phrase on a corresponding index server; store a plurality of shards of different phrase posting lists on a first index server; store a plurality of shards of different phrase posting lists on a second index server; and within each of the first and second index servers, for each shard of a phrase posting list, order the shard according to document identifiers of the documents included in the shard.
46. An indexing system for indexing documents in a document collection, the system comprising: a plurality of index servers; one or more memory devices configured store executable instructions; and one or more processors configured to execute the stored instructions to cause the system to: determine a phrase posting list associated with a first phrase, the phrase posting list identifying documents of the document collection associated with the first phrase; divide the phrase posting list for the first phrase into a plurality of shards, each shard identifying a subset of the documents identified by the posting list; store each shard of the phrase posting list for the first phrase on a corresponding index server; store a plurality of shards of different phrase posting lists on a first index server; store a plurality of shards of different phrase posting lists on a second index server; and within each of the first and second index servers, for each shard of a phrase posting list, order the shard according to document identifiers of the documents included in the shard. 48. The system of claim 46 , wherein the one or more processors are further configured to execute the stored instructions to cause the system to: associate a first set of index servers with a first tier of index servers; associate a second set of index servers with a second tier of index servers; and determine whether to store shards of the phrase posting list for the first phrase on first tier or the second tier based on a query processing cost for the phrase posting list.