Oracle Memory Architecture

Oracle Database includes several memory areas, each of which contains multiple sub-components.

The basic memory structures associated with Oracle Database include:

• System global area (SGA)The SGA is a group of shared memory structures, known as SGA components, that contain data and control information for one Oracle Database instance. All server and background processes share the SGA. Examples of data stored in the SGA include cached data blocks and shared SQL areas.
• Program global area (PGA)A PGA is a nonshared memory region that contains data and control information exclusively for use by an Oracle process. Oracle Database creates the PGA when an Oracle process starts.

  One PGA exists for each server process and background process. The collection of individual PGAs is the total instance PGA, or instance PGA. Database initialization parameters set the size of the instance PGA, not individual PGAs.

• User global area (UGA)The UGA is memory associated with a user session.