1. A method, comprising: at a computing system having one or more processors and memory storing one or more programs executed by the one or more processors: setting a respective reuse flag for a corresponding document in a plurality of documents based on a query-independent score associated with the corresponding document; performing a document crawling operation on the plurality of documents in accordance with the reuse flag for respective documents in the plurality of documents, including reusing a previously downloaded version of a respective document in the plurality of documents instead of downloading a current version of the respective document from a host computer in accordance with a determination that the reuse flag associated with the respective document meets a predefined criterion.