http://oracleinaction.com/tune-pga-i/

GA has two components : Tunable and untunable

– Untunable PGA : consists of

Context information of each session

Each open cursor

PL/SQL, OLAP or Java memory

This component of PGA can’t be tuned i.e. whatever is memory is needed it will be consumed else the operation fails.

– Tunable PGA : consists of memory available to SQL work areas (used by various sort operations)

 approx. 90% of PGA in DSS systems

 approx. 10% of PGA in OLTP systems