for the smaller table my simple update is working fine ..
when i fired the same update for the big tables ( 30000000 rows), its running for 10 hrs and failing with rollback segments.
can any one tell me what is the best way to complete this update.

they way i did for the samll table is

SQL> SELECT * FROM CUST_OLD;

CUST_ID NAME
——– ——–
111 AAA
222 BBB
333 CCC
444 DDD
111 AAA
222 BBB
555 EEE

7 rows selected.

SQL> SELECT * FROM CUST_NEW;

CUST_ID CUST_NEW
——– ——–
111 1111
222 2222
333 3333
444 4444
555 5555
666 6666
777 7777
888 8888
999 9999

SQL> UPDATE CUST_OLD A SET A.CUST_ID=
2 (
3 SELECT
4 B.CUST_NEW_ID
5 FROM
6 CUST_NEW B
7 WHERE
8 A.CUST_ID=B.CUST_ID
9 AND B.CUST_NEW_ID IS NOT NULL)
10 WHERE EXISTS
11 (SELECT C.CUST_ID FROM CUST_NEW C WHERE A.CUST_ID=C.CUST_ID) ;

7 rows updated.

SQL> SELECT * FROM CUST_NEW;

CUST_ID CUST_NEW
——– ——–
111 1111
222 2222
333 3333
444 4444
555 5555
666 6666
777 7777
888 8888
999 9999

9 rows selected.

SQL> SELECT * FROM CUST_OLD;

CUST_ID NAME
——– ——–
1111 AAA
2222 BBB
3333 CCC
4444 DDD
1111 AAA
2222 BBB
5555 EEE

7 rows selected.

thanks