System Global Area
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 contains the following data structures:
- Database buffer cache
- Redo log buffer
- Shared pool
- Java pool
- Large pool (optional)
- Streams pool
- Data dictionary cache
SGA is System Global Area is a group of shared memory structures. Components:
1. Shared Pool – (a) Library Cache: SQL is parsed and validated (b) Data Dictionary Cache: It is the meta data of the database.
2. Database Buffer Cache – Used for processing of data. Works on LRU algorithm.
3. Redo Log Buffer – It holds information about changes made to the database.
4. Java Pool – It is used for the Java related programs.
5. Large Pool – It is used by the backup and recovery operations.