Can somebody please simplify/tune this query?

Question

Hi,
Can somebody please simplify this query?
It's a little urgent. Currently this query is taking 10 mins to run.
Thank you so much,
vasu
Purpose:
First Child Case Ind =
Get Parent Case for this case.
Get all child cases for this parent case.
Arrange them in ascending order by create Date.
If the first case (eldest case) is the case in question,
then Y
else N
Query:
SELECT THIS_CASE.OBJID THIS_CASE_OBJID,
PARENT.PARENT_OBJID,
(CASE
WHEN PARENT.FIRST_CHILD_OBJID = THIS_CASE.OBJID
THEN 'Y'
ELSE 'N'
END) IS_FIRST_CHILD_CASE
FROM SA.TABLE_CASE THIS_CASE,
(SELECT PARENT_OBJID,
CHILD_OBJID FIRST_CHILD_OBJID
FROM (SELECT PARENT_CASE.OBJID PARENT_OBJID,
CHILD_CASE.OBJID CHILD_OBJID,
CHILD_CASE.CREATION_TIME,
ROW_NUMBER() OVER (PARTITION BY PARENT_CASE.OBJID
ORDER BY CHILD_CASE.CREATION_TIME ASC) ROW_NUM
FROM SA.TABLE_CASE PARENT_CASE,
SA.TABLE_CASE CHILD_CASE
WHERE PARENT_CASE.OBJID = CHILD_CASE.CASE_VICTIM2CASE
WHERE ROW_NUM = 1
) PARENT
WHERE THIS_CASE.CASE_VICTIM2CASE = PARENT.PARENT_OBJID(+)

Answer 1

Thank you for the quick reply.
Here is the explain plan:
Operation     Object Name     Rows     Bytes     Cost
SELECT STATEMENT Optimizer Mode=CHOOSE          2 M          32831
HASH JOIN OUTER          2 M     130 M     32831
TABLE ACCESS FULL     SA.TABLE_CASE     2 M     24 M     21550
VIEW          307 K     11 M     10130
WINDOW SORT PUSHED RANK          307 K     7 M     10130
MERGE JOIN          307 K     7 M     8666
TABLE ACCESS BY INDEX ROWID     SA.TABLE_CASE     307 K     4 M     1266
INDEX FULL SCAN     SA.IND_CASE_VICTIM2CASE     307 K          738
SORT JOIN          2 M     19 M     7400
INDEX FAST FULL SCAN     SA.PK_TABLE_CASE     2 M     19 M     785
Please let me know.
Thank you.