When using automatic PGA management mode,as you may know there is limitation on the size of the assigned memory indicated by :
- _pga_max_size Maximum size of the PGA memory for one process
- _smm_max_size maximum work area size per porcess
- _smm_px_max_size maximum work area size per parallel query
The calculation of this parameters depend on the value set for “pga_aggregate_target”. Also the memory bound is adjusted based on the workload.For more info on the algorithm please check this white paper .
Here is a quick view :