less07 undo tb
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7Copyright 2005, Oracle. All rights reserved.
Managing Undo Data
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7-2 Copyright 2005, Oracle. All rights reserved.
Objectives
After completing this lesson, you should be able to dothe following: Explain DML and redo data generation
Monitor and administer undo data Describe the difference between undo data and
redo data Configure undo retention
Guarantee undo retention Use the Undo Advisor
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7-3 Copyright 2005, Oracle. All rights reserved.
Data Manipulation
Data manipulation language (DML) consists of thefollowing SQL statements: INSERT
UPDATE DELETE MERGE
DML always executes as part of a transaction,which can be: Rolled back, using the ROLLBACK command Committed, using the COMMITcommand
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7-4 Copyright 2005, Oracle. All rights reserved.
Data Manipulation and Redo Data
Block 1 Block 3Block 2
SQL> COMMIT;
Redo log file
Verify that redo data is on the
disk and write COMMITrecord
Write verified, COMMITrecord written, control returned
SQL>
SQL> DELETE FROM employees WHERE salary > 10000;
SQL> UPDATE employeesSET salary = 5000
WHERE employee_id = 202;
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7-5 Copyright 2005, Oracle. All rights reserved.
Undo Data
Undo data is: A copy of original, premodified data Captured for every transaction that changes data
Retained at least until the transaction is ended Used to support:
Rollback operations Read-consistent and flashback queries
Recovery from failed transactions
User
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7-6 Copyright 2005, Oracle. All rights reserved.
Undo DataFull Notes Page
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Transactions and Undo Data
Each transaction is assignedto only one undo segment.
An undo segment can service
more than one transaction at atime.
Updatetransaction
Old value
New
valueUndo segment
Table
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Storing Undo Information
Undo information is stored in undo segments, whichare, in turn, stored in an undo tablespace. Undotablespaces:
Are used only for undo segments Have special recovery considerations May be associated with only a single instance Require that only one of them be the current
writable undo tablespace for a given instance atany given time
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Undo Data Versus Redo Data
Inconsistent reads in multiuser systems
Undo segments
Rollback, read-consistency,
and flashback
How to undo a change
Undo
Protectsagainst
Stored in
Used for
Record of
Recovery, to roll
forward databasechanges
Redo log files
Data loss
How to reproduce achange
Redo
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10/207-10 Copyright 2005, Oracle. All rights reserved.
Monitoring Undo
Undo usually requires little management. The areas tomonitor include: Free space in an undo tablespace
Snapshot too old errors
DBA
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Monitoring UndoFull Notes Page
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Configuring Undo Retention
DBA
UNDO_RETENTIONspecifies (in seconds) the amount of already committed undo information that is to beretained. The only time you must set this parameter iswhen: The undo tablespace has the AUTOEXTEND option
enabled You want to set undo retention for LOBs You want to guarantee retention
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Guaranteeing Undo Retention
Guarantee: 15 minutes
A transaction that generatesmore undo than what there
is space for will fail.
Undo data
SELECT statementsrunning 15 minutes or less
are always satisfied.
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Sizing the Undo Tablespace
Current table-space size
Undo consumption rate
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Using the Undo Advisor
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Summary
In this lesson, you should have learned how to: Explain DML and redo data generation Monitor and administer undo segments
Configure undo retention Guarantee undo retention Use the Undo Advisor
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Practice Overview:Managing Undo Segments
This practice covers the following topics: Calculating undo tablespace sizing to support a
48-hour retention interval
Modifying an undo tablespace to support a 48-hour retention interval
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