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Oracle Core: Essential Internals for DBAs and Developers by Jonathan Lewis

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Transactions and Undo

When you create a database, you have to create an undo tablespace (and if you’re using RAC, this is extended to one undo tablespace for each instance that will access the database). Unless you’re using old-style manual rollback management, Oracle will automatically create several undo segments in that tablespace and will automatically add, grow, shrink, or drop undo segments as the workload on the database changes.

Transaction management starts with, and revolves around, the undo segments. The segment header block, which (for undo segments) is the first block of the segment, contains a lot of the standard structures that you will see in the segment header block of other types of segment—the extent map and the extent control ...

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