You can use the flashback technology at the database, table, and transaction levels:

• Flashback database enables you to take the entire database to a past point in time (using flashback logs).

• Flashback drop lets you retrieve accidentally dropped tables and indexes (using the recycle bin).

• Flashback table lets you recover a table to a time in the past (using undo data).

• Flashback query lets you query and restore data rows to a point in time (using undo data).

The initialization parameter UNDO_RETENTION enables you to specify the length of time Oracle must retain undo information in the undo segments.

Default value:   900 (in seconds)

Modifiable:       ALTER SYSTEM

Range:             0 to 232 – 1

By default, Oracle doesn’t guarantee undo retention. Methods to specify Undo Guarantee:

o By using the RETENTION GUARANTEE clause when you create an undo tablespace:

o You can also use the ALTER TABLESPACE command:

o You can specify undo guarantee for the undo tablespace when you create the database.

Note: You can use the RETENTION NOGUARANTEE clause to turn off the guaranteed retention of undo information.

Note: The amount of time for which undo is retained for the Oracle Database for the current undo tablespace can be obtained by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT dynamic performance view.

Note: Use Oracle’s Undo Advisor to get approximate undo parameter values as well as suggestions regarding the sizing of your undo tablespace to successfully support flashback for a specified time.

• Oracle gives you a choice between using either clock time or the system change number (SCN) to specify exactly what time in the past you want to go back to.

• The SCN_TO_TIMESTAMP SQL function lets you convert an SCN to a calendar time (TIMESTAMP) value. There is a mapping granularity of three seconds.

• The TIMESTAMP_TO_SCN function converts a timestamp to its corresponding SCN.