alter system set memory_max_target=20G scope=both; ou scope=spfile
alter system set memory_target=17G scope=both;
Restartar a instância e ver o parâmetro de memória
SQL> show parameter memory
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
hi_shared_memory_address integer 0
memory_max_target big integer 1G
memory_target big integer 1G
shared_memory_address integer 0