The Oracle System Global Area (SGA) is a shared memory region in Oracle Database that contains data and control information for the database instance. It is used to improve database performance by reducing the amount of physical I/O operations that are required to access data, and by enabling efficient sharing of data between database sessions.
The SGA is divided into several components, each of which serves a specific purpose. The main components of the SGA are:
1. Buffer Cache: This component is responsible for caching frequently accessed data blocks from datafiles. When a user process requests data, Oracle first checks if the data block is available in the buffer cache. If it is, the data is returned from the buffer cache. If it is not, the data block is retrieved from disk and added to the cache.
2. Shared Pool: This component is used by Oracle to cache frequently used SQL statements, parsed statements or execution plans, PL/SQL procedures, function calls, shared cursors, and other data structures to share them across multiple database sessions. The shared pool also contains the library cache, which stores information about database objects required by SQL and PL/SQL statements.
3. Redo Log Buffer: This component is used to store information about changes made to the database, such as insert, update, and delete operations. This information is written to the redo log files to ensure that the database can be recovered in the event of a system failure.
4. Large Pool: This component is used to allocate memory for large memory allocations for certain operations like backup, recovery, and Parallel Execution Server.
5. Java Pool: This component is used to store session-specific Java objects and metadata for Java Virtual Machine.
6. Streams Pool: This component is used to hold streams pool buffers used for Oracle Streams technology.
Each component of the SGA is allocated according to the initialization parameters specified in the instance startup parameter file. These parameters determine the sizes and configurations of individual SGA components.
In summary, the SGA plays an important role in Oracle Database performance by improving system efficiency and reducing I/O operations required to access frequently used data. The various components of the SGA work together to ensure efficient memory allocation and data sharing across the database.