The default caching mechanism of operating system may not meet the caching needs of a database system. Because

  • Database may not store data inform of files that the operating system understands.
  • The data may be accessed in such a way that only the DBMS can predict.
  • The related blocks of a block of data is only known to the DBMS server.
  • DBMS may have a user specific policy when swapping data from cache

In database context, cache my refer to a space in memory of file system, that is dedicated to storing of certain information. The cache may contain query plans or the query results. All commercial databases implement the plan cache, whereas very few vendors support results cache, because of its complexity and resource usage.