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UNDO_RETENTION parameter not retrieving deleted data

UNDO_RETENTION=900

I am using dbms_flashback query. It is running successfully but the deleted data is not retrieved. Why would this happen?
The UNDO_RETENTION parameter determines how many seconds Oracle will keep undo in the UNDO tablespace, provided that there is enough room. If the transaction has committed, and the undo has not yet passed the UNDO_RETENTION time, but someone needs this space for a new transaction, then Oracle will use that space. You can monitor how many times this occurs by looking at the V$UNDOSTAT view, specifically the UNXPSTEALCNT column, which shows how many times unexpired undo space was stolen.

This was last published in June 2004

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