Undo are not destroyed immediately after the commit or rollback as they will be used in Read consistent mechanism. So Oracle uses Undo segments to restore the before image of update for those block from Undo segments to get the read consistent views But the blocks are getting updated by User B after some time. Now user A should get the read consistent version of the table at time t. User B start updating the same table at time t’. User A start the select cursor on the table at time t. Undo also provides us the read consistency to a user as he can only see the previous values until a transaction is committed.To explain, I would give one example So, it has preserve values for each of the columns of the row to be able to “Undo” the delete.Ĭonversely, the undo for an insert is a Delete of the row by ROWID – which does not have to preserve the values being deleted. The undo for a delete is more for than that for an insert.That is because the undo for a delete is actually the re-Insert of the whole row.
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