manual pga:

SORT_AREA_RETAINED_SIZE specifies (in bytes) the maximum amount of the user global area (UGA) memory retained after a sort run completes. The retained size controls the size of the read buffer, which Oracle uses to maintain a portion of the sort in memory. This memory is released back to the UGA, not to the operating system, after the last row is fetched from the sort space.

Oracle may allocate multiple sort spaces of this size for each query. Usually, only one or two sorts occur at one time, even for complex queries. In some cases, however, additional concurrent sorts are required, and each sort keeps its own memory area. If the shared server is used, allocation is to the SGA until the value inSORT_AREA_RETAINED_SIZEis reached. The difference betweenSORT_AREA_RETAINED_SIZEandSORT_AREA_SIZEis allocated to the PGA.

A sort area retained size will not influence whether a sort is done in RAM or not -- just

where the RESULTS of the sort will be stored.

sort_area_retained_size,在完成了所有排序所返回的结果集之后才会释放掉该内存区域.
因为这个参数并不用于sort. 因为在sort完成后,需要作sort join.而SORT_AREA_RETAINED_SIZE 用于sort  join.因此需要等返回后才能释放