9. A method implemented by at least one computer processor, the method comprising: caching a recurring query and associated document result entries in a cache; determining that a received query is the recurring query; maintaining an incremental index that includes a changed document index and an unchanged document index, the changed document index identifying changed documents that have changed since a previous time and the unchanged document index identifying unchanged documents that have not changed since the previous time; when processing the received query, computing the received query on the changed document index without computing the received query on the unchanged document index, wherein an updated document result entry identifying an individual changed document is obtained from the changed document index; and serving at least some of the associated document result entries from the cache and the updated document result entry in response to the received query.
9. A method implemented by at least one computer processor, the method comprising: caching a recurring query and associated document result entries in a cache; determining that a received query is the recurring query; maintaining an incremental index that includes a changed document index and an unchanged document index, the changed document index identifying changed documents that have changed since a previous time and the unchanged document index identifying unchanged documents that have not changed since the previous time; when processing the received query, computing the received query on the changed document index without computing the received query on the unchanged document index, wherein an updated document result entry identifying an individual changed document is obtained from the changed document index; and serving at least some of the associated document result entries from the cache and the updated document result entry in response to the received query. 12. The method of claim 9 , wherein the received query is in the cache when the received query is computed on the changed document index without computing the received query on the unchanged document index.