1. If the current extent has more free blocks then the next free block is allocated. 2. Otherwise, if the next extent expired then wrap in the next extent and return the first block. 3. If the next extent is not expired then get space from the UNDO tablespace. If a free extent is available then allocate it to the undo segment and return the first block in the new extent. 4. If there is no free extent available, then steal expired extents from offline undo segments. De-allocate the expired extent from the offline undo segment and add it to the undo segment. Return the first free block of the extent. 5. If no expired extents are available in offline undo segments, then steal from online undo segments and add the new extents to the current undo segment. Return the first free block of the extent. 6. Extend the file in the UNDO tablespace. If the file can be extended then add an extent to the current undo segment and then return the block. 7. Tune down retention in decrements of 10% and steal extents that were unexpired, but now expired with respect to the lower retention value. 8. Steal unexpired extents from any offline undo segments. 9. Try to reuse unexpired extents from own undo segment. If all extents are currently busy (they contains uncommitted information) go to the step 10. Otherwise, wrap into the next extent. 10. Try to steal unexpired extents from any online undo segment. 11. If all the above fails then return ORA-30036 unable to extend segment by %s in undo tablespace '%s'
参考文档：http://blog.51cto.com/tiany/1432894 Doc ID 1579779.1
来源：https://www.cnblogs.com/dayu-liu/p/9026200.html