Database Instance

A database instance contains a collection of memory structures and Oracle Database background processes.

• The main memory structures are the System Global Area (SGA) and the Program Global Areas (PGAs).
• The background processes operate on the stored data (data files) in the database and use the memory structures to do their work.

A database instance exists only in memory.

Oracle Database also creates server processes to handle the connections to the database on behalf of client programs, and to perform the work for the client programs.

For example, parsing and running SQL statements, and retrieving and returning results to the client programs.

These types of server processes are also referred to as foreground processes.