Order of execution plan

Below reference clearly explains the order of execution plan.

http://www.dba-oracle.com/t_order_sequence_sql_execution_explain_plans_steps.htm


Cross verify results with below examples:

Example:
Query:
select /*+ gather_plan_statistics */
e.first_name,d.department_name,j.job_title,j.job_id
from
hr.employees e,
hr.departments d,
hr.jobs j
where
e.job_id=j.job_id
and e.department_id=d.department_id
and j.job_title in ('Purchasing Clerk','Marketing Manager','Administration Assistant')
--and e.first_name in ('Sigal','Alexander','Jennifer','Susan')
;
SELECT *  FROM  TABLE(DBMS_XPLAN.DISPLAY_CURSOR(null,null,'ALLSTATS LAST')) ;

Plan:

Id	Operation	Name	Starts	E-Rows	A-Rows	A-Time	Buffers	OMem	1Mem	Used-Mem
0	SELECT STATEMENT		1		7	00:00.0	19
*  1	HASH JOIN		1	17	7	00:00.0	19	880K	880K	629K (0)
2	NESTED LOOPS		1		7	00:00.0	12
3	NESTED LOOPS		1	17	7	00:00.0	9
*  4	TABLE ACCESS FULL	JOBS	1	3	3	00:00.0	7
*  5	INDEX RANGE SCAN	EMP_JOB_IX	3	6	7	00:00.0	2
6	TABLE ACCESS BY INDEX ROWID	EMPLOYEES	7	6	7	00:00.0	3
7	TABLE ACCESS FULL	DEPARTMENTS	1	27	27	00:00.0	7

order of id execution: 4,5,3,6,2,7,1,0

It follows "Left-Right-Center" at every node from leaf to top in tree.

It follows post order traversal. Refer this.
https://www.tutorialspoint.com/python_data_structure/python_tree_traversal_algorithms.htm