oracle tune

https://oracle-base.com/articles/8i/explain-plan-usage

https://docs.oracle.com/cd/E11882_01/server.112/e41084/sql_elements006.htm#SQLRF51098

https://oracle-base.com/articles/9i/dbms_xplan

https://oracle-base.com/articles/11g/real-time-sql-monitoring-11gr1

select /*+MONITOR*/ from dual;