MRU (fetch-and-discard) replacement strategy is used for table scanning on heaps. This strategy places pages into the cache just before the wash marker, as shown in Figure 7-3.
Figure 7-3: MRU strategy places pages just before the wash marker
Fetch-and-discard is most often used for queries where a page is needed only once by the query. This includes:
Most table scans in queries that do not use joins
One or more tables in a join query
Placing the pages needed only once at the wash marker means that they do not push other pages out of the cache.
The fetch-and-discard strategy is used only on pages actually read from the disk for the query. If a page is already in cache due to earlier activity on the table, the page is placed at the MRU end of the cache.
Figure 7-4: Finding a needed page in cache