Explain the purpose of the System Global Area (SGA) in Oracle database architecture.

The System Global Area (SGA) in Oracle database architecture is a crucial component responsible for managing and storing shared memory structures and data buffers required for the functioning of the Oracle database instance. It serves as a shared memory area accessible to all server processes associated with the Oracle instance.

Here's a technical breakdown of the purpose of SGA and its components:

  1. Buffer Cache (Database Buffer Cache):
    • The Buffer Cache is a portion of the SGA that stores copies of data blocks read from data files on disk.
    • When a user process requests data, Oracle first checks if the required data is already available in the Buffer Cache. If it is, Oracle retrieves the data from the cache, avoiding the need to read from disk, which is slower.
    • If the data is not found in the Buffer Cache, Oracle fetches it from the data files on disk and places a copy of the data block into the Buffer Cache for future access.
  2. Shared Pool:
    • The Shared Pool is another significant component of the SGA that stores shared memory structures such as parsed SQL statements, execution plans, and PL/SQL code.
    • When a user submits a SQL query, Oracle first checks if the parsed representation of the query exists in the Shared Pool. If it does, Oracle reuses the existing parsed representation, saving the overhead of re-parsing the query.
    • Similarly, frequently executed PL/SQL code and execution plans of SQL statements are cached in the Shared Pool to optimize performance and reduce CPU overhead.
  3. Redo Log Buffer:
    • The Redo Log Buffer in the SGA is responsible for temporarily holding redo log entries before they are written to the redo log files on disk.
    • Redo log entries contain a record of changes made to the database, such as insertions, updates, and deletions.
    • The Redo Log Buffer ensures that these changes are durably recorded in the redo log files, providing the necessary information for database recovery and transaction rollback.
  4. Large Pool (Optional):
    • The Large Pool is an optional component of the SGA used for allocating memory for large database operations such as backup and restore operations, parallel execution buffers, and I/O server processes.
    • It helps in reducing contention for memory resources in the Shared Pool and Buffer Cache by segregating large memory allocations into a separate area.
  5. Java Pool (If Oracle JVM is enabled):
    • The Java Pool, present in the SGA, is used to manage memory for Java objects and sessions when Oracle JVM (Java Virtual Machine) is enabled in the database.
    • It provides memory allocation and management capabilities specific to Java stored procedures, triggers, and other Java-based functionalities within the Oracle database.