Categories of UNDO in Oracle

Back to: Oracle DBA Tutorials

Categories of UNDO in Oracle

In this article we will learn about Categories of Undo in Oracle. Please read our previous article discussing Undo Retention Period in Oracle with examples.

Active:

• Uncommitted undo information. We had issued an update statement and didn’t issue a commit after the update statement.
• If a user wants to roll back or the transaction has failed,
• Uncommitted undo information is never overwritten.
• If data is not committed, the uncommitted data will be stored in the undo segments; it takes up space and cannot be overwritten. We need this uncommitted transaction if we need to roll back.

Unexpired:

• It is the committed undo information, but we need to store it for read consistency.
• Old undo information with an age that is less than the undo retention period.
• This is retained for consistent read and Oracle flashback operations.

Expired:

• This is the expired undo information. Old committed undo information older than the current undo retention period is said to be expired, and its space is available to be overwritten by new transactions.

These are the categories present in the UNDO tablespace and the UNDO segments. In the next article, I will discuss the Flashback Table and Flashback Query in Oracle. In this article, I explain the Categories of UNDO in Oracle with examples. I hope you enjoy this Oracle.