If a row has changes that were not committed at the

• If a row has changes that were not committed at the start of query,
the database server constructs a read- consistent image of the row
by retrieving the before image of the changes from the undo
segment and applying the changes to a copy of the row in memory.

Read Consistency
Transaction Read Consistency
• Read consistency is always provide from SQL statement. However,
you can request read consistency for a read-only transaction by
issuing the following command at the beginning of the transaction:
SET TRANSACTION READ ONLY;
• Or, you can request read consistency for a transaction performing
DML by issuing the following command at the beginning of the
transaction:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
• In either case, the database server provides data that is consistent
from the start of the transaction. Using SERIALIZABLE can have a
negative impact on performance.

Types of Undo Segments
• SYSTEM : Used for objects in the SYSTEM tablespace
• Non-SYSTEM : Used for objects in other tablespace :
• Auto Mode : Requires an UNDO tablespace
• Manual Mode :
• Private : Acquired by a single instance
• Public : Acquired by any instance
• Deferred : Used when tablespaces are taken offline immediate,
temporary , or for recovery

Types of Undo Segments
SYSTEM Undo Segment
• The SYSTEM undo segment is created in the SYSTEM tablespace
when a database is created.
• This undo segment can be used only for changes made to objects
in the SYSTEM tablespace.
• The SYSTEM undo segment exists and works the same in both
manual and auto mode.

Types of Undo Segments
Non-SYSTEM Undo Segments
• A database that has multiple tablespaces needs at least one Non-
SYSTEM undo segment for manual mode or one UNDO tablespace
for auto mode.
Manual Mode
• In manual mode, a non-system undo segment, which is created by
the database administrator, can be used for changes made to
objects in any non-system tablespace.
• There are two types of non-system undo segments.

Types of Undo Segments
Private
• Private undo segments are segments that are brought online by an
instance because they are listed in the parameter file.
• They can be brought online explicitly by issuing an ALTER
ROLLBACK SEGEMENT command.

Types of Undo Segments
Public
• Public undo segments form a pool of undo segments available in a
database.
• Public undo segments are normally used with the database Parallel
Server to create a pool of undo segments that can be used by any
of the Parallel Server instances.
• Deferred Undo Segments Deferred undo segments may be created
when a tablespace is brought offline.
• They are used to roll back transactions when the tablespace is
brought back online.
• They are dropped automatically when they are no longer needed.
• Because deferred undo segments are maintained by the database
server, no maintenance is required on your part.

Automatic Undo Management : Concepts
• Undo data is managed using an UNDO tablespaces.