Q

Does Oracle overwrite rollback information for uncommitted transactions?

If the undo_retention parameter is set to 900 sec and after that period if unexpired (uncommitted) data is overwritten by other transactions in some undo segment, then how does rollback work?

If in Oracle 9i or 10g the undo_retention parameter is set to 900 sec and after that period if unexpired (uncommitted) data is overwritten by other transactions in some undo segment, then how does rollback work in that case? As undo has been overwritten which was supposed to be used for rollback?
Oracle will not overwrite the rollback information for uncommitted transactions, even if the UNDO_RETENTION time has passed. Oracle will overwrite this information only for committed transactions. So if a transaction runs for longer than 900 seconds in your example, you will still be able to rollback that transaction.
This was first published in June 2007

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