Oracle Database Administration: The Essential Refe

System Global Area (SGA)

The System Global Area (SGA) is the shared memory area used by the instance to store information that must be shared between the database and user processes. The main components of the SGA include the database buffer cache, shared pool, and redo log buffers. The database buffer cache contains actual copies of data blocks from the database. When you update a table, the information is first modified in the data buffer and is later written to disk. The shared pool contains the SQL and PL/SQL statements being executed; every SQL statement executed is stored in the shared pool, which allows Oracle to reuse all the information it has generated about the statement. This reuse produces significant improvements in processing speeds for users in an online transaction processing (OLTP) environment who are executing the same SQL statements. The redo log buffer collects the redo entries for the online redo logs prior to their being flushed to disk.