What is the difference between undo and redo in oracle

Failure, redo can restore those. Redo is due to the fact that every commit, the modification of the data is immediately written to the online redo, but it does not necessarily write changes to the data file at the same time. Because the number. It has been submitted, but only exists in the online log file, so you need to recover the data from the online log files, re-apply, so that the data has changed in the number.

The reason for undo is that, in order to improve efficiency when Oracle is running, the DBWR process writes dirty blocks to the data if the user has not yet made a commit, but does not have much free memory. File in order to free up valuable memory for use by other processes. That's why you need to undo. Only the first redo apply success, to ensure that the undo datafile inside the things are correct, and then can rollback. The purpose of undo is to restore the system to the state before the system crashes before the shutdown , and then the redo is to ensure the consistency of the system.

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If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email. If you find any instances of plagiarism from the community, please send an email to: info-contact alibabacloud. A comprehensive suite of global cloud computing services to power your business. Undo: Undo, that is, cancel the previous operation. Redo: Redo, perform the previous operation again.

What is Redo Redo records transaction logs, divided into online and archived. For the purpose of recovery. What is Undo REDO is to re-implement your operation, and undo, instead, is to undo the actions you have made, such as if you have a transaction execution failure or you regret it, You need to fallback to the operation with the rollback command. Features of the data in undo: 1.

Is the backup before data modification, mainly to ensure the user's read consistency 2. Undo is a record of a transaction before it is committed. On the other hand, redo is an entry in redo log files that holds a group of change vectors. What is Undo in Oracle -Definition, Functionality 2. What is Redo in Oracle -Definition, Functionality 3. Oracle database creates and manages information that is used to roll back.

Here, this information refers to the records on transactions before they are committed. These records are also called undo. Undo tablespace is a specific tablespace that only stores undo information. Each database contains zero or more undo tablespaces.

In Oracle, there is an automatic undo management mode. In most Windows applications this keyboard shortcut functions as Redo, reversing a previous Undo. The maximum number of undo actions you can perform by default is Windows users can adjust the limit to a smaller number by tweaking the Windows Registry.

The redo function restores any actions that were previously undone using an undo. The redo log is a disk-based data structure used during crash recovery to correct data written by incomplete transactions.

MySQL writes to the redo log files in a circular fashion. Note: Oracle Enterprise Manager 9. This mode protects the database from both instance and media failure, but may require additional hardware resources.

When you enable block change tracking in Oracle 11g, Oracle tracks the physical location of all database changes for incremental backups.