Introduction

As an Oracle Database administrator, managing the undo tablespaces effectively is essential. The undo tablespace is responsible for storing the data required to undo any modifications made to the database. However, Over time, the undo tablespace can accumulate much data, leading to performance issues. In this article, we will discuss why we need to cleanup the undo tablespaces and provide a step-by-step guide on performing this task.

Understanding Undo Tablespaces

Before we dive into the cleanup process, let’s first understand what undo tablespaces are and how they work.

What is an Undo Tablespace?

An undo tablespace is a logical storage container in an Oracle Database that contains undo data. The undo data is used to roll back or undo changes made to the database.

How Does Undo Tablespace Work?

Whenever a transaction modifies the data in the database, the corresponding undo data for the modification is stored in the undo tablespace. In case the transaction needs to be rolled back, the stored undo data is utilized to reverse the changes made to the database.

Why is Undo Tablespace Cleanup Important?

As we mentioned previously, the undo tablespace has the potential to accumulate a significant amount of data over time. This can lead to various performance issues, including slow database operations, longer backup and recovery times, and higher storage costs. It is, therefore, crucial to perform regular undo tablespace cleanup to mitigate these issues.

Steps to Perform Undo Tablespace Cleanup

Here are the steps to undo tablespace cleanup:

1. Determine the Size of the Undo Tablespace
2. Identify the Undo Segments
3. Switch to a New Undo Tablespace
4. Monitor the Undo Tablespace
5. Drop the Old Undo Tablespace

Step 1: Determine the Size of the Undo Tablespace

Before performing the cleanup, it is essential to determine the size of the undo tablespace. This can be done using the following SQL command:

SELECT   TABLESPACE_NAME, SUM (BYTES) / 1024 / 1024 TS_SIZE_MB
FROM     DBA_UNDO_EXTENTS
GROUP BY TABLESPACE_NAME;

Output:

Determine the Size of the Undo Tablespace using SQL Developer

To identify the size of undo tablespace using SQL Developer navigate to your schema from connections panel –> right click and choose manage database

Determine the Size of the Undo Tablespace using TOAD

To identify the size of undo tablespace using TOAD navigate to Database –> Administer –> Tablespaces

Step 2: Identify the Undo Segments

Next, identify the undo segments that consume the most space in the undo tablespace. We can get this information by executing the following SQL command:

SELECT   tablespace_name,segment_name, SUM (BYTES) / 1024 / 1024 TS_SIZE_MB
FROM     DBA_UNDO_EXTENTS
GROUP BY segment_name,tablespace_name
order by SUM (BYTES) desc;

Output:

Step 3: Switch to a New Undo Tablespace

Once you have identified the undo segments that consume the most space, switch to a new undo tablespace. This can be done using the following SQL command:

ALTER SYSTEM SET UNDO_TABLESPACE = UNDO_TBS2; 

Step 4: Monitor the Undo Tablespace

After switching to the new undo tablespace, monitor its usage to ensure that it is not becoming too large. This can be done using the following SQL command:

SELECT tablespace_name, sum(bytes)/1024/1024 size_mb, sum(maxbytes)/1024/1024 max_size_mb
FROM dba_data_files
WHERE tablespace_name = 'UNDO_TBS2'
GROUP BY tablespace_name;

Output:

Step 5: Drop the Old Undo Tablespace

Once you are satisfied that the new undo tablespace is working correctly, drop the old undo tablespace. This can be done using the following SQL command:

DROP TABLESPACE UNDO_TBS2 INCLUDING CONTENTS AND DATAFILES; 

Conclusion

Performing regular undo tablespace cleanup is crucial for maintaining the performance and stability of an Oracle database. By following the step-by-step guide outlined in this article, you can identify and manage the undo segments that consume the most space, switch to a new undo tablespace, and drop the old one.

FAQs

How often should I perform undo tablespace cleanup?

It is recommended to undo tablespace cleanup regularly, depending on the amount of activity in the database. A good rule of thumb is to perform it at least once a month.

Can I perform undo tablespace cleanup during business hours?

It is recommended to undo tablespace cleanup during non-business hours to avoid any potential disruptions to database operations.

Can I automate the undo tablespace cleanup process?

Yes, it is possible to automate the undo tablespace cleanup process using scripts or third-party tools.

Can I recover data after performing undo tablespace cleanup?

No, once the undo tablespace cleanup process is completed, the undo data is permanently deleted, and the changes cannot be undone. Therefore, backing up your database before performing any cleanup operations is crucial.