Oracle Database Undo Management

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Questions and Answers

What action occurs when all extents have been consumed in the undo segment?

The database triggers an error message.

The transaction wraps back into the first extent or requests a new extent. (correct)

The transaction terminates immediately.

The undo tablespace is resized automatically.

What type of tablespace can undo segments only exist in?

Permanent tablespace

Undo tablespace (correct)

Temporary tablespace

Bigfile tablespace

Which configuration does the Database Configuration Assistant (DBCA) automatically create?

A bigfile undo tablespace

A permanent data file

A smallfile undo tablespace (correct)

A temporary undo tablespace

What is a major distinction between undo data and redo data?

Undo data restores changes, while redo data re-applies them. Signup and view all the answers

What parameter sets the low threshold value of undo retention for an auto-extending undo tablespace?

UNDO_RETENTION Signup and view all the answers

In a high-volume OLTP environment, what can cause contention in an undo tablespace?

Having many short concurrent transactions Signup and view all the answers

What happens to the undo retention period for auto-extending tablespaces during an active query?

It is tuned to be slightly longer than the active query's duration. Signup and view all the answers

What may need to be performed if a database utilizes Flashback operations?

Administrative tasks to manage undo data and space. Signup and view all the answers

What is the primary purpose of undo data in an Oracle Database server?

To allow rollback of uncommitted data Signup and view all the answers

What happens if the original data for a read-consistent query is no longer available?

The query results in a 'Snapshot too old' error Signup and view all the answers

How are undo segments managed within the Oracle Database server?

They automatically grow and shrink as needed Signup and view all the answers

What is the function of a flashback query in the context of undo data?

To return data as it existed at a specific point in the past Signup and view all the answers

What is the consequence of a failed transaction in an Oracle Database?

The associated undo data can help recover from failed operations Signup and view all the answers

Which of the following statements accurately describes undo segments?

They consist of data blocks grouped into extents Signup and view all the answers

How can users check transaction assignment to undo segments?

By checking the V$TRANSACTION view Signup and view all the answers

What occurs when a transaction's assigned undo segment runs out of space?

The transaction acquires additional space from the next extent Signup and view all the answers

What is the role of uncommitted undo information?

It supports currently running transactions that may need to roll back. Signup and view all the answers

What happens to expired undo information when space is required by an active transaction?

It is overwritten to free up space for active transactions. Signup and view all the answers

How does guaranteed retention affect the default undo behavior?

It enforces the retention of undo information even at the risk of transactions failing. Signup and view all the answers

What is the significance of the UNDO_RETENTION setting in automatic undo management?

It is ignored unless retention guarantee is enabled. Signup and view all the answers

Which command is used to set undo retention guarantee for an undo tablespace?

ALTER TABLESPACE undotbs1 RETENTION GUARANTEE; Signup and view all the answers

What characterizes committed undo information?

It is retained to meet the undo retention interval. Signup and view all the answers

Which attribute controls the guaranteed retention of undo information?

RETENTION GUARANTEE Signup and view all the answers

What type of undo information is never overwritten?

Uncommitted undo information Signup and view all the answers

What will occur if an attempt is made to set retention guarantee on a non-undo tablespace?

An error will indicate that retention can only be specified for undo tablespaces. Signup and view all the answers

What are two reasons for changing the undo tablespace to a fixed size?

To support Flashback operations and prevent excessive growth. Signup and view all the answers

What can happen if the undo tablespace is not sized appropriately?

Users will experience DML failures and 'snapshot too old' errors. Signup and view all the answers

What is a primary reason for using temporary tables in a database?

To facilitate faster operations compared to non-temporary tables. Signup and view all the answers

What happens to undo for operations on temporary tables?

It is logged to the redo log but is not persisted beyond the session. Signup and view all the answers

What is one of the benefits of enabling temporary undo?

It enhances the performance of redo log parsing. Signup and view all the answers

How does temporary undo affect DML operations on temporary tables in a standby database?

It allows DML operations using the Active Data Guard option. Signup and view all the answers

What is a recommended practice for managing the undo tablespace size?

Regularly running a full workload to allow it to grow appropriately. Signup and view all the answers

What must be done for DDL operations that create temporary tables?

They must be issued on the primary database. Signup and view all the answers

What happens when you enable temporary undo for a session using an ALTER SESSION statement?

It only affects the session where it is enabled. Signup and view all the answers

Which initialization parameter must be set for the temporary undo feature to be available?

COMPATIBLE set to at least 12.1.0.0.0 Signup and view all the answers

What does the TXNCOUNT column in the V$TEMPUNDOSTAT view represent?

Total number of transactions bound to temp undo segments. Signup and view all the answers

How often are the rows in the V$TEMPUNDOSTAT view updated?

Every 10 minutes. Signup and view all the answers

What does a high value in the MAXCONCURRENCY column indicate?

A high number of transactions executed concurrently on temporary objects. Signup and view all the answers

What information does the NOSPACEERRCNT column in the V$TEMPUNDOSTAT view provide?

Total instances where there was no space left for temporary undo. Signup and view all the answers

What is the consequence of using temporary objects when temporary undo is disabled for a session?

Temporary undo cannot be enabled for the session until it is restarted. Signup and view all the answers

Study Notes

Undo Data

Oracle Database server saves the old values of data when a process changes them
Undo data is used for rollbacks, read-consistent queries, and flashback queries
Read-consistent queries require undo data to return results as they existed at the start of the query
Flashback queries require undo data to return results as they existed at a specific point in the past
Undo data is used for recovering failed transactions

Undo Segments

Undo segments hold the old values of data changed during a transaction
Transactions are assigned to specific undo segments
Undo segments are automatically created as needed, and are made up of data blocks
Undo segments automatically grow and shrink based on transaction activity

Undo Tablespace

Undo segments are stored in a specialized tablespace called an undo tablespace
Undo tablespaces are automatically created by the DBCA as smallfile tablespaces
Undo tablespaces can also be created as bigfile tablespaces
Undo tablespaces can be stored in multiple data files to avoid contention in high-volume OLTP environments

Undo vs Redo

Undo data is used to undo changes to data
Redo data is used to reapply changes to data
Undo block changes are also written to the redo log

Automatic Undo Management

Oracle Database server automatically manages undo data and space in a dedicated undo tablespace
Undo retention period is automatically tuned based on the longest-running query
Undo retention period can be set using the UNDO_RETENTION initialization parameter

Undo Information Categories

Uncommitted undo information is used for rollback and failed transactions
Committed undo information is used for meeting the undo retention interval
Expired undo information is overwritten when space is needed by an active transaction

Guaranteed Retention

Undo retention settings can be enforced even if they cause transactions to fail
RETENTION GUARANTEE tablespace attribute can be set using an ALTER TABLESPACE command

Fixed-size Undo Tablespace

Fixed-size undo tablespaces can be used to support flashback operations or prevent the tablespace from growing
A large enough size should be chosen to avoid DML failures or "Snapshot too old" errors

Temporary Undo

Enables reduced undo storage in undo tablespaces
Reduces the size of the redo log
Enables DML operations on temporary tables in a physical standby database with Oracle Active Data Guard
Enabled for a specific session or for the entire database using ALTER SESSION or ALTER SYSTEM statements

V$TEMPUNDOSTAT View

Provides statistics related to the temporary undo log
Displays a histogram of statistical data collected in 10-minute intervals
Provides information on transaction counts, concurrent transactions, undo block counts, and errors

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Description

Test your knowledge on Oracle Database's undo data, segments, and tablespaces. This quiz covers key concepts like how undo data is used for transactions and the role of undo segments in ensuring data integrity. Perfect for those looking to master Oracle's data management features.