You can simplify and improve the way PGA memory is allocated by enabling automatic PGA memory management. In this mode, Oracle Database dynamically adjusts the size of the portion of the PGA memory dedicated to work areas, based on an overall PGA memory target explicitly set by the DBA. To enable automatic PGA memory management, you must set the initialization parameter PGA_AGGREGATE_TARGET. For new installations, PGA_AGGREGATE_TARGET and SGA_TARGET are set automatically by the database configuration assistant (DBCA), and MEMORY_TARGET is zero. That is, automatic memory management is disabled. Therefore, automatic tuning of the aggregate PGA is enabled by default. However, the aggregate PGA does not grow unless you enable automatic memory management by setting MEMORY_TARGET to a nonzero value.
Oracle Database Reference for more information about the PGA_AGGREGATE_TARGET initialization parameter
Oracle Database Performance Tuning Guide for descriptions of how to use PGA_AGGREGATE_TARGET in different scenarios
HASH_AREA_SIZE has been deprecated and you should use PGA_AGGREGATE_TARGET instead.
SORT_AREA_SIZE has been deprecated and you should use PGA_AGGREGATE_TARGET instead.