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The following
sections describe these areas of the SGA. For a more detailed discussion of
performance and the SGA, see ???How Oracle Uses the System Global Area??? in
Chapter 7.
Database buffer cache
The database buffer cache holds blocks of data retrieved from the database. This
buffer between the users??™ requests and the actual datafiles improves the performance
of the Oracle database. If a piece of data can be found in the buffer cache (for example,
as the result of a recent query), you can retrieve it from memory without the
overhead of having to go to disk. Oracle manages the cache using a least recently used
(LRU) algorithm. If a user requests data that has been recently used, the data is more
likely to be in the database buffer cache; data in the cache can be delivered immediately
without a disk-read operation being executed.
When a user wants to read a block that is not in the cache, the block must be read
and loaded into the cache. When a user makes changes to a block, those changes are
made to the block in the cache.


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