how to update the previous rows with last_modified date column having null values?

Question

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).

Thanks

Answer 1

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.

Answer 2

If your rows were modified "recently enough" , you might still map their ora_rowscn to their approximate modification TIMESTAMP using SCN_TO_TIMESTAMP :

UPDATE MY_TABLE
    SET Last_Modified = SCN_TO_TIMESTAMP(ora_rowscn)
    WHERE Last_Modified IS NULL;

This is not a magic bullet though. To quote the documentation:

• The usual precision of the result value is 3 seconds.
• The association between an SCN and a timestamp when the SCN is generated is remembered by the database for a limited period of time. This period is the maximum of the auto-tuned undo retention period, if the database runs in the Automatic Undo Management mode, and the retention times of all flashback archives in the database, but no less than 120 hours. The time for the association to become obsolete elapses only when the database is open. An error is returned if the SCN specified for the argument to SCN_TO_TIMESTAMP is too old.

If you try to map ora_rowscn of rows outside the allowed window, you will get the error ORA-08181 "specified number is not a valid system change number" .