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Chapter 11: MANAGING UNDO DATA (Explain the Purpose of Undo (The preupdate…
Chapter 11: MANAGING UNDO DATA
Explain the Purpose of Undo
Purpose
Reverse
the effects of DML statements
The preupdate version is written in
Rollback
segment
Retain for
backward
compatibility
Create rollback segments in the
SYSTEM
tablespace at
database creation time
SEGMENT_TYPE: ROLLBACK
Undo
segment
Should use
Create in normal running
SEGMENT_TYPE: TYPE 2 UNDO
Type of rollback
Rollback for transaction
Use data from undo segments to construct an image of data as
before the transaction
Permanent
and
Published
to all users
Flashback query
Query the database at
some time in the past
Temporary
and ONLY
visible
to the
session running the query