Multiple index based information retrieval system

1. A computer-implemented method comprising:

assigning each phrase identified in a document collection a phrase number based on frequency of occurrence of the phrase in the document collection, wherein each indexed document has a document identifier;

creating a phrase-sharded index for a search engine by, for each identified phrase:

assigning the phrase to a server of a plurality of index servers based on a hash of the assigned phrase number, and

storing a posting list of identifiers of documents of the document collection that contain the phrase on the assigned index server;

identifying, using the index, documents responsive to a search query; and

providing information about the identified documents to a requestor of the search query.

2. The method of claim 1 , further comprising for at least a phrase of the identified phrases:

selectively partitioning the posting list for the phrase into at least a first portion including identifiers of higher ranked documents and a second portion including identifiers of lesser ranked documents, the partitioning being based on a relevance score for a document identified in the posting list indicating the document's relevance to the phrase;

storing the first portion in a primary index on the assigned index server; and

storing the second portion in a secondary index on the assigned index server.

3. The method of claim 2 , wherein a maximum quantity of documents is stored in the primary index.

4. The method of claim 2 , further comprising storing, for each identifier of a document in the first portion of the posting list, relevance attributes of the document.

5. The method of claim 2 , wherein the search query relates to the phrase and identifying documents responsive to the search query includes:

determining a result using the primary index and not the secondary index;

determining the result includes a sufficient set; and

ranking the documents in the result.

6. The method of claim 1 , wherein most frequently occurring phrases have lower phrase numbers.

7. An information retrieval system for retrieving information from a corpus of documents, the system comprising:

a primary index server system comprising a primary index, the primary index including primary phrase posting lists, each primary phrase posting list being associated with a phrase; and

a secondary index server system comprising a secondary index, the secondary index including secondary phrase posting lists, each secondary phrase posting list being associated with a primary phrase posting list in the primary index, and including documents that contain the phrase that is associated with the primary phrase posting list in the primary index and which have relevance scores less than the relevance score of a lowest ranked document in the primary phrase posting list for the phrase,

wherein the primary index server system comprises multiple machines, and wherein each phrase is assigned an identification number and has a primary phrase posting list located on one of the machines.

8. The information retrieval system of claim 7 , wherein the assigned identification number is based on frequency of occurrence of the phrase in the corpus of documents.

9. The information retrieval system of claim 7 , wherein phrases are assigned to a machine of the multiple machines based on a hash of the assigned identification number.

10. The information retrieval system of claim 7 , wherein the primary phrase posting lists include up to a maximum number of documents of the corpus that contain the phrase.

11. A non-transitory computer-readable medium having stored thereon instructions that, when executed by at least one processor, cause a computing system to perform operations including:

assigning each phrase identified in a document collection a phrase number based on frequency of occurrence of the phrase in the document collection, wherein each document has a respective document identifier; and

creating a phrase-sharded index for a search engine by, for each identified phrase:

assigning the phrase to a server of a plurality of index servers based on a hash of the assigned phrase number, and

storing a posting list of identifiers of documents of the document collection that contain the phrase on the assigned index server;

identifying, using the index, documents responsive to a search query; and

providing information about the identified documents to a requestor of the search query.

12. The computer-readable medium of claim 11 , wherein the operations further comprise, for at least a phrase of the identified phrases:

selectively partitioning the posting list for the phrase into at least a first portion including identifiers of higher ranked documents and a second portion including identifiers of lesser ranked documents, the partitioning being based on a relevance score for a document identified in the posting list indicating the document's relevance to the phrase;

storing the first portion in a primary index on the assigned index server; and

storing the second portion in a secondary index on the assigned index server.

13. The computer-readable medium of claim 12 , wherein a maximum quantity of documents is stored in the primary index.

14. The computer-readable medium of claim 12 , further comprising storing, for each identifier of a document in the first portion of the posting list, relevance attributes of the document.

15. The computer-readable medium of claim 12 , wherein the search query relates to the phrase and identifying documents responsive to the search query includes:

determining a result using the primary index and not the secondary index;

determining the result includes a sufficient set; and

ranking the documents in the result.

16. The computer-readable medium of claim 12 , wherein the first portion has a maximum quantity of document identifiers and the second portion has a remainder of the document identifiers.

17. The computer-readable medium of claim 11 , wherein most frequently occurring phrases have lower phrase numbers.