In 2003, Google introduced a “supplemental index” as a way of showing more documents to users. Most webmasters will probably snicker about that statement, since supplemental docs were famous for refreshing less often and showing up in search results less often. But the supplemental index served an important purpose: it stored unusual documents that we would search in more depth for harder or more esoteric queries. For a long time, the alternative was to simply not show those documents at all, but this was always unsatisfying—ideally, we would search all of the documents all of the time, to give users the experience they expect. Read more
