How to use Timestamp_to_scn and Scn_to_timestamp i

问题:

I have this as a result of the query:

select cast(to_date(a.start_time,'mm/dd/yyyy hh:mi:ss pm') as timestamp) date_of_call,
ora_rowscn from calling_table a where rownum <= 10;

       DATE_OF_CALLING          ORA_ROWSCN

26-JUL-13 12.29.28.000000000 PM 8347567733892
26-JUL-13 12.29.35.000000000 PM 8347567733892
26-JUL-13 12.29.35.000000000 PM 8347567733892
26-JUL-13 12.29.38.000000000 PM 8347567733892
26-JUL-13 12.29.44.000000000 PM 8347567733892
26-JUL-13 12.29.47.000000000 PM 8347567733892
26-JUL-13 12.29.48.000000000 PM 8347567733892
26-JUL-13 12.29.48.000000000 PM 8347567733892
26-JUL-13 12.29.48.000000000 PM 8347567733892
26-JUL-13 12.29.56.000000000 PM 8347567733892

But when I try to convert this timestamp into scn using the function timestamp_to_scn, I am getting the following error:

ORA-08180: no snapshot found based on specified time ORA-06512: at "SYS.TIMESTAMP_TO_SCN", line 1 08180. 00000 - "no snapshot found based on specified time" *Cause: Could not match the time to an SCN from the mapping table. *Action: try using a larger time.

And when I am using scn_to_timestamp on ora_rowscn to convert that column into a timestamp, I am getting the following error:

ORA-08181: specified number is not a valid system change number ORA-06512: at "SYS.SCN_TO_TIMESTAMP", line 1 08181. 00000 - "specified number is not a valid system change number" *Cause: supplied scn was beyond the bounds of a valid scn. *Action: use a valid scn.

What is it that I am doing wrong?

回答1:

You're trying to look too far back. You can only convert to and from SCNs that are in the redo/flashback window maintained by your system. Once changes age out then the mapping is lost.

This is explained in the documentation:

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.

Bear in mind these are part of Oracle's internal mechanism, and so are of limited use to us; though they are useful for flashback queries of course - again within the same window.

回答2:

Set UNDO_MANAGEMENT to AUTO, and UNDO_RETENTION to value that will cover the period of your longest query back in time. Also set RETENTION GARANTEE to prevent UNDO from being overwritten.

For Oracle 10g you can not flashback longer than 5 days. This is a hard coded limit. For Oracle 11g there is no limitation.