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OCP Instructors Guide for Oracle DBA Certification

Chapter 5 - Oracle Database Objects

Oracle9i - Database Managed Undo Segments

In Oracle9i, administrators have their choice of continuing to manage rollback segments on their own (manual undo management) or configuring the database to manage its own before image data (automatic undo management). Oracle refers to system managed before image segments as undo segments.

Administrators must create a tablespace to hold undo segments by using the new UNDO keyword in the tablespace create statement:

DATAFILE 'undotbs_1a.f'

The following initialization parameters are used to activate automatic undo management:

  • undo_management  – AUTO configures the database is to use automatic undo segments. MANUAL configures the database to use rollback segments.

  • undo_tablespace – Specifies the tablespaces that are to be used to hold the undo segments. The tablespace must be created using the UNDO keyword. If no tablespace is defined, Oracle will select the first available undo tablespace. If no undo tablespaces are present in the database, Oracle will use the system rollback segment during startup. This value can be set dynamically by using the ALTER SYSTEM statement.

  • undo_retention – specifies the amount of time that Oracle attempts to keep undo data available. This parameter becomes important when the Oracle9i flashback query option is used.

You cannot create database objects in undo tablespaces. It is reserved for system-managed undo data. The view dba_undo_extents  can be accessed to retrieve information relating to system managed undo data. For those of us familiar with v$rollstat, it is still available and the information reflects the behavior of the undo segments in the undo tablespace.

The above text is an excerpt from:

OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques

ISBN 0-9744355-3-8

by Christopher T. Foot

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