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.
The SGA is made up of three required components and three optional components.
Required SGA Components:
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1)Shared Pool:Caches the most recently used SQL statements that have been issued by database users.
2)Database Buffer Cache: Caches the data that has been most recently accessed by database users.
3)Redo Log Buffer: Stores transaction information for recovery purposes
Optional SGA Components:
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1)Java Pool:Caches the most recently used Java objects and application code when
Oracle’s JVM option is used.
2)Large Pool:Caches data for large operations such as Recovery Manager (RMAN)
backup and restore activities and Shared Server components
3)Streams Pool:Caches the data associated with queued message requests when
Oracle’s Advanced Queuing option is used.
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