Compare two tables and update the mismatch column in Oracle SQL

I have a requirement to compare with one table and update another table if there are mismatches. Table LEGACY_CUST_INFO has correct CUST_PRIVILEGE_NUMBER. So, I need to findout the wrong CUST_PRIVILEGE_NUMBER in second table CUST_INFO and update with the right CUST_PRIVILEGE_NUMBER from first table LEGACY_CUST_INFO. ROW_ID in both the tables match are an exact match. Both tables have approximately 20 million records.

Below are the two table structures

Table 1 Name : LEGACY_CUST_INFO

ROW_ID	CUST_PRIVILEGE_NUMBER

Table 2 Name : CUST_INFO

ROW_ID CUST_PRIVILEGE_NUMBER

I am getting performance issues when I check with the below statements. Is there a better way to create a table with mismatch records and update the column CUST_PRIVILEGE_NUMBER in CUST_INFO table ?
Below is my code and logic.

1. I am creating a table - EXACT_RECORDS with exact match records.
2. Deleting the exact match records from LEGACY_CUST_INFO table.
3. Updating the CUST_PRIVILEGE_NUMBER based on LEGACY_CUST_INFO table.

CREATE TABLE EXACT_RECORDS
AS
(SELECT A.* FROM LEGACY_CUST_INFO A, CUST_INFO B WHERE A.ROW_ID = B.ROW_ID AND A.CUST_PRIVILEGE_NUMBER = B.CUST_PRIVILEGE_NUMBER);

DELETE FROM LEGACY_CUST_INFO WHERE ROW_ID IN (SELECT ROW_ID FROM EXACT_RECORDS);

UPDATE CUST_INFO B SET B.CUST_PRIVILEGE_NUMBER = (SELECT A.CUST_PRIVILEGE_NUMBER FROM LEGACY_CUST_INFO A WHERE A.ROW_ID = B.ROW_ID);

I know this logic is very basic. Could you help in better way so that 20 millions records are updated in less than 30 minutes ?

I suggest to using a merge statement containing a query, returning prepared not matched records for update.

merge into cust_info dst
using (
select l.row_id, l.cust_privilege_number
from legacy_cust_info lci
inner join cust_info ci on ci.row_id = lci.row_id
where ci.cust_privilege_number != lci.cust_privilege_number
) src
on (dst.row_id = src.row_id)
when matched then 
  update set dst.cust_privilege_number = src.cust_privilege_number;

Also consider create indexes for your tables:

CREATE INDEX legacy_cust_info#rid#cpn#idx ON LEGACY_CUST_INFO (row_id, cust_privilege_number);
CREATE INDEX cust_info#row_id#idx ON CUST_INFO (row_id);

UPD.
You can estimate the time by running the nested select without merging its contents into the destination table:

select count(1) from (
  select l.row_id, l.cust_privilege_number
   from legacy_cust_info lci
   inner join cust_info ci on ci.row_id = lci.row_id
   where ci.cust_privilege_number != lci.cust_privilege_number
)

UPD2. To considering NULLs in ci.cust_privilege_number simply add alternative condition into the query:

where (
  ci.cust_privilege_number != lci.cust_privilege_number 
  or (
   ci.cust_privilege_number is null 
   and lci.cust_privilege_number is not null
  )
)

Indexing helps to retrieve data faster. Check the below query: -

For row_ID

CREATE INDEX idx_legacy_cust_info_row_id ON LEGACY_CUST_INFO (ROW_ID);
CREATE INDEX idx_cust_info_row_id ON CUST_INFO (ROW_ID);

For single update statement using a join between the two tables to update the mismatched CUST_PRIVILEGE_NUMBER. check the following query: -

UPDATE CUST_INFO CI
SET CI.CUST_PRIVILEGE_NUMBER = (
   SELECT LCI.CUST_PRIVILEGE_NUMBER
   FROM LEGACY_CUST_INFO LCI
   WHERE LCI.ROW_ID = CI.ROW_ID
)
WHERE CI.CUST_PRIVILEGE_NUMBER <> (
   SELECT LCI.CUST_PRIVILEGE_NUMBER
   FROM LEGACY_CUST_INFO LCI
   WHERE LCI.ROW_ID = CI.ROW_ID
);