Since Oracle Database 10g, the Oracle JVM has a new memory model for sessions that connect to the database through a dedicated server. The basic memory structures associated with Oracle include:

• System Global Area (SGA)

  The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. The SGA is shared by all server and background processes. Examples of data stored in the SGA include cached data blocks and shared SQL areas.

• Program Global Areas (PGA)

  A PGA is a memory region that contains data and control information for a server process. It is nonshared memory created by Oracle when a server process is started. Access to the PGA is exclusive to the server process. There is one PGA for each server process. Background processes also allocate their own PGAs. The total PGA memory allocated for all background and server processes attached to an Oracle instance is referred to as the aggregate PGA.

The simplest way to manage memory is to allow the database to automatically manage and tune it for you. To do so, you set only a target memory size initialization parameter (MEMORY_TARGET) and a maximum memory size initialization parameter (MEMORY_MAX_TARGET), on most platforms. The database then tunes to the target memory size, redistributing memory as needed between the SGA and aggregate PGA. Because the target memory initialization parameter is dynamic, you can change the target memory size at any time without restarting the database. The maximum memory size serves as an upper limit so that you cannot accidentally set the target memory size too high. Because certain SGA components either cannot easily shrink or must remain at a minimum size, the database also prevents you from setting the target memory size too low.