Fetch-and-discard (MRU) scans

When a scan uses MRU replacement policy, buffers are handled in a special manner when they are read into the cache by asynchronous prefetch. First, pages are linked at the MRU end of the chain, rather than at the wash marker. When the query accesses the page, the buffers are re linked into the pool at the wash marker. This strategy helps to avoid cases where heavy use of a cache flushes prefetched buffers linked at the wash marker before they can be used. It has little impact on performance, unless large numbers of unneeded pages are being prefetched. In this case, the prefetched pages are more likely to flush other pages from cache.