What is Undo and Redo in DBA?

What is Undo and Redo in DBA?

Opposite is where Oracle records how to reverse a deal (one or more DMLs in a transaction)

Redo is where Oracle records how to replay a transaction

Undo and Upgrade are published to as the deal continues, not merely at the end of the transaction

(imagine a deal that comprises of 1million single-row places, each unique place is published to undo and redo)

Undo segments

Oracle dynamically makes and falls Opposite sections based on deal volume

An undo section comprises of several extents. As a deal develops beyond the present stage, a new stage may be allocated

One undo section supports several dealings but a deal cannot interval several undo segments

After COMMIT the undo details is maintained for undo_retention or autotuned_undo_retention.

At the end of the preservation interval, the undo is removed, the stage is terminated

Undo retention

Oracle may autotune the undo retention

If the datafile(s) for the effective undo tablespace are set to autoextend OFF, Oracle instantly uses the datafile to the maximum and disregards undo_retention

If the datafile(s) are set to autoextend ON, Oracle autotunes undo_retention to suit question lengths

Check V$undostat for this information

Undo and Study Consistency

Oracle’s performance of MultiVersionReadConsistency uses a customer interval being able to look at the undo produced by another session

A interval may need to look at the before picture of details because the details has been customized (and may even have been commited) by another session

It imitations the present form of the prevent it is studying and is applicable the undo for that prevent for making its read reliable version

Flashback Entirely sustained by studying from Undo


Read Reliability with READ COMMITTED is at *statement* stage by default

A interval operating several concerns may each read a different edition by standard because Study Dedicated is required for each statement

(This does mean that if you have a PLSQL prevent operating the same SQL many periods, each performance can see a different form of the data– if the details are customized by another interval between accomplishments of the SQL !)

An interval can decide to set it’s ISOLATION LEVEL to SERIALIZABLE meaning that every question recognizes the same form of data

This performs only in abbreviation operating concerns and with few changes to the details or read only details.

SERIALIZABLE can upgrade details given that the same details were modified and committed by another interval after the beginning (else you get ORA-08177)

READ ONLY does not allow plenty of your energy and effort interval for making changes


When a deal is in improvement, it is recognized by the Transaction Deal with, Opposite section, port and sequence

The ITL port in the prevent headlines contains the referrals (address) to the Undo

The SCN is allocated at commit time (therefore a deal does not begin with an SCN)

Temp Undo

12c also allows short-term undo

Normally, changes to GTT produce undo which needs to be published to undo segments

With 12c temperature undo, those undo records are also, like the real changes, short-term and can be removed when the commit is issued

Thus, the undo does not need to be published to hard drive (remember details in a GTT is not noticeable to another interval, so there is no need to continue to persist the undo). You can get the oracle jobs by joining the best oracle training to make your career in this field.