A system global area (SGA) is a group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users. Consequently, the SGA is sometimes called the shared global area.
An SGA and Oracle processes constitute an Oracle instance. Oracle automatically allocates memory for an SGA when you start an instance, and the operating system reclaims the memory when you shut down the instance. Each instance has its own SGA.
The SGA is read/write. All users connected to a multiple-process database instance can read information contained within the instance's SGA, and several processes write to the SGA during execution of Oracle.
- Database buffer cache
- Redo log buffer
- Shared pool
- Java pool
- Large pool (optional)
- Streams pool
- Data dictionary cache
- Other miscellaneous information
Part of the SGA contains general information about the state of the database and the instance, which the background processes need to access; this is called the fixed SGA. No user data is stored here. The SGA also includes information communicated between processes, such as locking information.
If the system uses shared server architecture, then the request and response queues and some contents of the PGA are in the SGA.
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