I have a loader table in which the feed updates and inserts records for every three hours. A few set of records show Null values for the last_modified date even though I have a merge which checks for last_modified date column to sysdate. For future purpose, I set the last_modified to sysdate and enabled Not NULL constraint.Is there any way where we can rectify for these set of records alone to have the last_modified date with the correct timestamp (the records should have the last_modified date with the date when the insert/update is done).
No, the last modification time is not stored in a row by default. You have to do that yourself like you are doing now, or enable some form of journaling. There is no way to correct any old records where you have not done so.