when trying to allocate a new extent for SMU in UNDO tablespace :
1. A new extent will be allocated from the undo tablespace.
2. If failed, ( no free space exists in the tablespace ) acquire expired extent from another undo segment.
This involves shrinking from the other segment and adding to the current undo segment.
3. If failed, reclaim expired extents from the current transaction's commited changes.
4. If failed, try allocating more space by datafile extention ( if autoextend ON ).
5. If failed, try reuse an unexpired extent from the current undo segment.
6. If failed, try stealing an unexpired extent from another undo segment.
7. If all failed, report an "Out-Of-Space" error.
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