oracle explain plan simple example

EXPLAIN PLAN
   SET STATEMENT_ID = 'st1'
   INTO my_plan_table
 FOR
SELECT last_name FROM employees;

SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());

SELECT PLAN_TABLE_OUTPUT 
  FROM TABLE(DBMS_XPLAN.DISPLAY('MY_PLAN_TABLE', 'st1','TYPICAL'));

https://docs.oracle.com/cd/B28359_01/server.111/b28274/ex_plan.htm#i16938

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-explain-the-explain-plan-052011-393674.pdf

https://blogs.oracle.com/sql/how-to-create-an-execution-plan