SORT AREA SIZE is a parameter which set the sort area size of the pga memory.
alter session set sort_area_size = 102400;
It will have no effect if the WORKAREA_SIZE_POLICY is set to auto.
The sort area size control how much memory will be dynamically allocated at runtime in order to satisfy a sort request. After the sort is completed, this memory will be released entirely.
The sort area size are not a permanently allocation of memory.