22. An indexing system for indexing documents of 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: receive a query for information from the document collection, wherein the query includes a first phrase and a second phrase; access a plurality of index servers to retrieve identifiers of documents on a phrase posting list associated with the first phrase, wherein the phrase posting list identifies documents of the document collection associated with the first phrase, and is divided into a plurality of shards, each shard identifying a subset of the plurality of the documents identified by the posting list, and each shard being stored on a corresponding index server of the plurality of index servers; access a plurality of the index servers to retrieve identifiers of documents on a phrase posting list associated with the second phrase, wherein the phrase posting list identifies documents of the document collection associated with the second phrase, and is divided into a plurality of shards, each shard identifying a subset of the plurality of the documents identified by the posting list, and each shard being stored on a corresponding index server of the plurality of index servers; and serve a response to the query, wherein the response is based on information in the phrase posting list associated with the first phrase and is based on information in the phrase posting list associated with the second phrase.
22. An indexing system for indexing documents of 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: receive a query for information from the document collection, wherein the query includes a first phrase and a second phrase; access a plurality of index servers to retrieve identifiers of documents on a phrase posting list associated with the first phrase, wherein the phrase posting list identifies documents of the document collection associated with the first phrase, and is divided into a plurality of shards, each shard identifying a subset of the plurality of the documents identified by the posting list, and each shard being stored on a corresponding index server of the plurality of index servers; access a plurality of the index servers to retrieve identifiers of documents on a phrase posting list associated with the second phrase, wherein the phrase posting list identifies documents of the document collection associated with the second phrase, and is divided into a plurality of shards, each shard identifying a subset of the plurality of the documents identified by the posting list, and each shard being stored on a corresponding index server of the plurality of index servers; and serve a response to the query, wherein the response is based on information in the phrase posting list associated with the first phrase and is based on information in the phrase posting list associated with the second phrase. 30. The system of claim 22 , 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 on the first tier or the second tier based on a query processing cost for the phrase posting list.