About System Global Area Memory

The System Global Area (SGA) is a memory area that contains data shared between all database users such as buffer cache and a shared pool of SQL statements. SGA is allocated in memory when an Oracle Database XE instance is started. The SGA contains the following subcomponents:

Buffer Cache. Functions as the buffer to store any data being queried or modified. All user processes connected to the database share access to the buffer cache. The buffer cache helps avoid repeated access from physical disk, a time consuming operation.
Shared Pool. Caches operational information and code that can be shared among users. For example:
- SQL statements are cached so that they can be reuse.
- Information from the data dictionary, such as user account data, table and index descriptions, and privileges, is cached for quick access and reusability.
- Stored procedures are cached for faster access.
Redo Log Buffer. Improves performance by caching redo information (used for instance recovery) until it can be written at once and at a more opportune time to the physical redo log files that are stored on disk.
Large Pool. Functions as an optional area for buffering large I/O requests for various server processes