On Commit Refresh FAQS

1. What is On Commit Refresh in Oracle?

On Commit Refresh in Oracle refers to the automatic refreshing of a materialized view whenever a commit operation occurs on the base tables. This ensures that the materialized view is immediately updated to reflect the latest committed changes in the base tables.

 

2. How does On Commit Refresh work?

When a commit occurs after a data modification (insert, update, delete) on the base tables, Oracle automatically triggers a refresh of the materialized view. The refresh happens after the commit is completed, ensuring that the materialized view stays synchronized with the committed data.

 

3. How do I enable On Commit Refresh for a materialized view?

To enable On Commit Refresh for a materialized view, specify the REFRESH ON COMMIT clause during the materialized view creation:

CREATE MATERIALIZED VIEW mv_name

REFRESH ON COMMIT

AS

SELECT column1, column2

FROM base_table;

This ensures that the materialized view will automatically refresh whenever a commit occurs on the base tables.

 

4. What types of refresh methods are available with On Commit Refresh?

With On Commit Refresh, Oracle typically uses Fast Refresh if possible. If Fast Refresh is not feasible, Oracle will use Complete Refresh. The refresh is determined based on the configuration and availability of materialized view logs on the base tables.

 

5. When should I use On Commit Refresh?

Use On Commit Refresh when:

• You need the materialized view to always reflect the most recent committed data.
• You want automatic synchronization after each commit without manual intervention.
• Data changes in the base tables are incremental, and real-time updates are needed.

 

6. What are the benefits of On Commit Refresh?

• Real-time updates: The materialized view is updated immediately after a commit, ensuring it always reflects the latest data.
• Automatic refresh: No need for manual refresh or scheduling.
• Transactional consistency: Ensures the materialized view remains synchronized with committed data.

 

7. What are the disadvantages of On Commit Refresh?

• Performance overhead: Frequent commits can trigger constant refreshes, which might affect performance.
• Locking issues: The refresh process may cause locking on the base tables or materialized view, impacting concurrent access.
• Scalability concerns: For large data sets or systems with high transaction rates, On Commit Refresh may be inefficient and slow.

 

8. How does Fast Refresh work with On Commit Refresh?

If materialized view logs are present on the base tables, Fast Refresh will be used when a commit occurs. Fast Refresh only applies the incremental changes (inserts, updates, deletes) to the materialized view. If Fast Refresh is not possible, Oracle will fall back to Complete Refresh, which rebuilds the entire materialized view.

 

9. How can I monitor On Commit Refresh?

You can monitor the refresh activity using views like DBA_MVIEW_REFRESH_TIMES to check the status of your materialized views and track the refresh times:

SELECT * FROM DBA_MVIEW_REFRESH_TIMES

WHERE mview_name = 'YOUR_MVIEW_NAME';

This helps in identifying when the materialized view was last refreshed.

 

10. Can I avoid the refresh if not necessary?

No, with On Commit Refresh, the refresh happens automatically after every commit. If the materialized view is not needed to be refreshed after every commit, you should consider other refresh methods like On Demand or Scheduled Refresh to have more control over when the refresh happens.

 

11. Is On Commit Refresh suitable for systems with high transaction volumes?

On Commit Refresh might not be suitable for systems with high transaction rates, as frequent refreshes can significantly impact performance. In such cases, other refresh strategies, such as On Demand or Scheduled Refresh, may be more appropriate to avoid excessive overhead.

 

12. What happens if I commit changes to base tables that are not part of the materialized view?

The On Commit Refresh will trigger a refresh only if the committed changes affect the materialized view’s underlying base tables. If the committed data does not affect the materialized view, no refresh will occur.

 

13. Can I combine On Commit Refresh with other refresh methods?

No, On Commit Refresh operates independently of other refresh methods like Scheduled Refresh or On Demand. However, you can configure materialized views to use other refresh methods (such as Fast or Complete) in conjunction with the On Commit trigger.

 

14. How does On Commit Refresh affect materialized view performance?

While On Commit Refresh ensures the materialized view is always up-to-date, it can introduce performance overhead, particularly in systems with frequent commits. This is because each commit will trigger a refresh operation, which can slow down overall database performance, especially if the materialized view is large or complex.

 

15. Can On Commit Refresh be used with materialized views based on remote tables?

Yes, On Commit Refresh can be used with materialized views based on remote tables. However, be mindful that this may introduce latency or performance issues due to the network and remote database access, especially if the remote table is frequently updated.

 

These FAQs cover the essential details regarding Oracle On Commit Refresh for materialized views. If you need more specifics, feel free to ask!