Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 13 RECOVERY
Next, let’s determine what the employee’s salary used to be, one hour ago. This is
done with the use of the clause:
Next, let’s formulate an update command to change the employee’s salary back to
the original value. Notice the use of the clause once again:
Flashback Versions
If a row of data is changed multiple times, the clause can be used to
search for prior versions of the row in the undo segments. and
are pseudocolumns that tell us when each version was created and which
transaction created it. The absence of these values indicates the time prior to the window
specified in the query. In the following example, you can see the original value of the
employee’s salary ($17,000), the transaction that changed the value to $18,000, and the
transaction that reversed the change: