Oracle Core: Essential Internals for DBAs and Developers

Undo Complexity

Undo is more complicated than redo. Most significantly, any process may, in principle, need to access any undo record at any time to “hide” an item of data that it is not yet supposed to see. To meet this requirement efficiently, Oracle keeps the undo records inside the database in a special tablespace known, unsurprisingly, as the undo tablespace; then the code has to maintain various pointers to the undo records so that a process knows where to find the undo records it needs. The advantage of keeping undo information inside the database in “ordinary” data files is that the blocks are subject to exactly the same buffering, writing, and recovery algorithms as every block in the database—the basic code to manage undo blocks is ...