19. The system of claim 16 , wherein the revised web crawl interval of a document is smaller than the document's content update interval and larger than half of the document's content update interval.
19. The system of claim 16 , wherein the revised web crawl interval of a document is smaller than the document's content update interval and larger than half of the document's content update interval. 20. The system of claim 19 , wherein the search engine system dynamically adjusts the revised web crawl interval of the document after retrieving a new copy of the document's content from a respective host.