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Oracle uses special structures called undo records to help provide automatic statement-level read consistency All data for a single query will come from a single point in time Only data committed when a query begins will be seen by the query any changes made by other transactions after a query begins won t be seen by the query once it begins executing If a transaction is modifying data, Oracle will write a before-image of the table data to its undo records For example, if you update the salary of an employee from 10,500 to 11,000, the undo record will store the old salary value of 10,500 When a query begins execution, Oracle will determine the current system change number, which identifies the order in which transactions occurred in the database.

When data blocks are read for this query, Oracle will only use data blocks with the SCN that it determined for this query When it encounters data blocks for the query with a more recent SCN, Oracle will automatically go to the undo segments and reconstruct the data from the undo records stored there Any changes made by other transactions during the query s execution will have more recent SCNs, and are disregarded, guaranteeing that only consistent data is returned for the query at hand Oracle s undo records are stored in the undo tablespace specified at database creation time The undo tablespace will always hold the before-image of table data for users if other transactions have updated it since a query began.

Undo data is used for the following purposes: Providing read consistency for SQL queries Rolling back unwanted active transactions Recovering terminated transactions Analyzing older data by using Flashback Query Recovering from logical corruptions using the Flashback features Remember that the undo data remains in the undo tablespace even after a database shutdown This makes Oracle s undo management valuable for activities beyond providing read consistency and rolling back transactions By using the Flashback features and undo data together, you can fix logical errors and query past data..

Figure 11-6 The class diagram of the system s top-level classes The class diagram looks incomplete, because there are no relationships between any of the classes I didn t forget the relationships: There simply are none I designed the system so that all its top-level classes are decoupled from each other The only classes missing from the diagram are those related to the infrastructure: the LifecycleCoordinators, the builder, and the binder, which I ll discuss shortly The classes are all pretty simple, but the class diagram tells you almost nothing about how the system is really structured, because you can t see how the parts interact when the system is running A more useful way to show the system is with a wiring diagram, as shown in Figure 11-7 The diagram depicts the system in the cruising state, which is the normal running state entered after system startup.

Automatic Undo Management takes the entire issue of sizing and allocation of undo segments from the DBA s hands and makes it Oracle s responsibility. As the DBA, all you have to do is create an adequately sized tablespace (the undo tablespace) for storing undo information. Oracle will dynamically create undo (rollback) segments and adjust their number to match the instance s workload requirements. The database will automatically allocate and deallocate undo segments to match the transaction throughput in the instance.

Proper undo management means that necessary undo information is not overwritten by newer undo data. By setting the appropriate size for the undo tablespace and the undo_retention interval, you can increase the chance that long-running queries can complete without receiving the snapshot-too-old error. It also ensures that critical Flashback features can retrieve the older data they are seeking.