Undo records are used for
- Database recovery
- Gives consistency read operation
- When a roll back statement is issued just roll back the part of the transaction
- Using oracle flashback features recover the logical corruptions
- Using oracle flashback query analyse data as an earlier point
Managing Space and Undo Data in Oracle Database 12C contains Undo records and are used for making the undo changes that are performed in the database, when roll back statement is issued for transaction that are unconditional. Amid recovery of the database, undo records will undo the uncommitted changes applied from the redo log to the data files. It provides read consistency by maintaining the user records who are accessing the database information at the same time another client is altering it. Oracle database stores the before image of a data in an undo segment in the designated undo table space while performing the initialization parameters. Undo server many supports roll back to undo changes that were made by uncommitted transactions. Undo provides data read consistency in the ACID model and they are also very crucial in flash back operations.
Acid model contain the key features of relational databases that is associated with the accompanying values
- Transaction
- Atomic
- Consistent
- Isolated
- Durable
A unit of work is said to be a transaction.In other words,a transaction is a sequence of related work and exchange of information maintained in a single unit.
An atomic activity is one that adequately happens at the same time in any programming part. An atomic activity can't halt in the center it either happens totally, or it doesn't happen by any stretch of the imagination. No reaction of an atomic activity are unmistakable until the activity is finished.
Consistent means that each client seems to be in consistency view about a data,that includes physical alterations done by the client individual transaction and other client transaction.
Each transaction should not be distributed to other sessions or other transactions.
Durability contains the entire durability components that are associated with each and every transactions.While functioning on nearby environment the durability of a transaction is said to be consistent.