Undo Extents

In Undo Segments there are three types of extents, they are

Unexpired Extents - Undo data whose age is less than the undo retention period.
Expired Extents - Undo data whose age is greater than the undo retention period.
Active Extents - Undo data that is part of the active transaction.

The sequence for using extents is as follows,

1. A new extent will be allocated from the undo tablespace when the requirement arises. As undo is being written to an undo segment, if the undo reaches the end of the current extent and the next extent contains expired undo then the new undo (generated by the current transaction) will wrap into that expired extent, in preference to grabbing a free extent from the undo tablespace free extent pool.

2. If this fails because of no available free extents and we cannot autoextend the datafile, then Oracle tries to steal an expired extent from another undo segment.

3. If it still fails because there are no extents with expired status then Oracle tries to reuse an unexpired extent from the current undo segment.

4. If even that fails, Oracle tries to steal an unexpired extent from another undo segment.

5. If all the above fails, an Out-Of-Space error will be reported.

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