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The Undo segment is where the before images of changed rows are stored. Oracle will manage your undo segments for you, but you need to determine how large to make the tablespace that the Undo segment is stored in. The size that you make this depends on the length of time that you want the undo information to be available to you. If you look at Figure 3-7, you will see how OEM helps you determine the length of time that Undo can be retained based on the system activity and Undo tablespace. This is in the Configuration section for an instance. If the tablespace is not the correct size, it can be changed using the Storage feature of OEM, which you will see later in this chapter. Undo segments are covered further in Chapter 5.
If you choose to implement user-managed rollback segments, then these can be managed in the Storage section of OEM by choosing Rollback Segments and then selecting the segment name that you want to manage. If you look at Figure 3-7, you will see a Rollback Segment named System. By the way, the System rollback segment will always exist but should never be used as a rollback segment for user processes.
FIGURE 3-7. Enterprise Manager Automatic Undo Management view
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