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undo

Posted on 2006-11-02
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Last Modified: 2012-06-22
"truncate table, alter table, select statements will not generate any undo data.."

What does it means " will not generate any undo data"?

please explain?
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Question by:dbaseek
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by:dbaseek
ID: 17861204
i would appreciate that
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Accepted Solution

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rbrooker earned 500 total points
ID: 17861254
Hi,

undo data is used to rollback a statement prior to that statement being committed.

these commands are "non-recoverable" commands. they contain an implicit commit, once thay have completed, they cannot be rolled back.

if you truncate a table by mistake, you may be able to use flashback to get that table back, but this is not the same as rolling back a transaction.
alter table is a DML statement, these also contain an implicit commit, once the change has been made, it cannot be rolled back.  you can issue another change to undo what you did, eg, drop column after add column, but these are 2 statements, the second is not a rollback of the first.
selects do not create undo as they are not making changes to data contained within the table.

good luck :)
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Expert Comment

by:Nabilbahr
ID: 34108620
I altered a table and lost all the data on that table , anyway to undo this
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