Rollback Segments Wizard (Oracle)
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Rollback segments manage all transactions in your Oracle databases. A transaction is a read, modify, and write cycle for an Oracle database. A rollback entry is made for all transactions unless a particular clause is specified. So, a rollback segment is a transaction log that consists of a pre-update image value and transaction status, among other things. The rollback segments maintain read consistency among concurrent users in a database and if the transaction fails for any reason, the old image is taken from the rollback segment. By maintaining a history of data changes, rollback segments can rollback uncommitted transactions so that data is rolled back to the prior state. SYS owns all rollback segments no matter who created them and are not accessible to users, just Oracle.
Oracle, it should be mentioned, strongly recommends that you use automatic undo management to simplify managing databases. Tuning rollback segments is a manual process that has largely been deprecated by Oracle, but it is supported for backward compatibility reasons.
The Rollback Segment Wizard lets you:
- Name the rollback segment and to place it online or off-line.
- Place the rollback segment on a tablespace.
- Specify the initial next and optimal extent size as well a the minimum and maximum number of extents that should be allocated to the rollback segment.
- Note: This wizard is not available if auto-UNDO management is enabled.
- Tip: Make sure enough rollback segments exist on a database to handle the imposed workload. One rule of thumb is to create one rollback segment for every four concurrent users.
To Open the Rollback Segment Wizard
- On the Navigator, find the datasource where you want to create a rollback segment and expand the Storage node.
- Right-click the Rollback Segments node, and select New.
The table that follows describes the fields you may encounter as you complete the wizard.
| Required Information | Description |
|---|---|
|
What is the name of the rollback segment? |
Self-explanatory. |
|
Should this rollback segment be made public? |
Yes - A public rollback segment can be brought online by any instance in a parallel server. Public rollback segments form a pool of rollback segments that can be used by any instance that needs one. No - This is the default. A private rollback segment can only be acquired by the instance specifying the segment in its initialization file. |
|
Do you want to place the rollback segment to be online following its creation? |
Online - To be useful, a rollback segment must be online. Offline - You may want to take rollback segments offline if you want to take a tablespace offline and it contains rollback segments that you want to keep from being used. |
|
On which tablespace do you want to place this rollback segment? |
Self-explanatory. Oracle suggests that you create one or more tablespaces specifically to hold all rollback segments. This way, the data contained in the rollback segments is held apart from other data types. |
|
What extent sizes do you want to assign to this rollback segment? |
Initial size (KB) Next size (KB) Optimal size (KB) Null/Default |
|
What are the minimum and maximum number of extents to allocate to the rollback segment? |
Minimum Maximum |