Replacement strategies determine where the page is placed in cache when it is read from disk. The optimizer decides on the cache replacement strategy to be used for each query. The two strategies are:
Fetch-and-discard, or MRU replacement, strategy links the newly read buffers at the wash marker in the pool.
LRU replacement strategy links newly read buffers at the most-recently used end of the pool.
Cache replacement strategies can affect the cache hit ratio for your query mix:
Pages that are read into cache with the fetch-and-discard strategy remain in cache a much shorter time than queries read in at the MRU end of the cache. If such a page is needed again (for example, if the same query is run again very soon), the pages will probably need to be read from disk again.
Pages that are read into cache with the fetch-and-discard strategy do not displace pages that already reside in cache before the wash marker. This means that the pages already in cache before the wash marker will not be flushed out of cache by pages that are needed only once by a query.
See “Specifying the cache strategy” on page 45 and “Controlling large I/O and cache strategies” on page 47 in the book Performance and Tuning: Optimizer for information on specifying the cache strategy in queries or setting values for tables.
