What is Undo and Redo in DBA?

1
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
2

• 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

3

• 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

4
Undo and Study Consistency

• Oracles performance of MultiVersionReadConsistenc
  y uses a customer interval being able to look at
  the undo produced by another session
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5
Transactions

• 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

6
Temp Undo

• 12c also allows short-term undo
• Normally, changes to GTT produce undo which needs
  to be published to undo segments

7

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