•When a transaction starts, Oracle will assign the corresponding transaction to only one undo segment. It is not possible for the undo data generated by one transaction to cut across multiple undo segments.
•It is need to know that undo segments are not fixed in size. So if a transaction does manage to fill its undo segment, Oracle will automatically add another extent to the segment so that the transaction can continue.
•It is possible for multiple transactions to share one undo segment, but in normal running this should not occur.Oracle will automatically generate new undo segments on demand, in an attempt to
ensure that it is never necessary for transactions to share undo segments.
•If Oracle has found it necessary to extend its undo segments or to generate additional segments, when the workload drops Oracle will shrink and drop the segments, again automatically.
Related Documents:
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How Undo Space Allocation Works
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