SORT_AREA_SIZE specifies in bytes the maximum amount of memory Oracle will use for a sort. After the sort is complete, but before the rows are returned, Oracle releases memory down to the size specified by the SORT_AREA_RETAINED_SIZE parameter. After the last row is returned, Oracle releases the remainder of the memory
Increasing SORT_AREA_SIZE size improves the efficiency of large sorts. Multiple allocations never occur. Only one memory area of SORT_AREA_SIZE exists for each user process at any time.
Larger values of SORT_AREA_SIZE permit more sorts to be performed in memory. If more space is required to complete the sort than will fit into the memory provided, then temporary segments on disk are used to hold the intermediate sort runs.

alter system set sort_area_size = 10485760 scope = spfile;