On Commit Refresh

In Oracle, an On Commit Refresh for materialized views means that the materialized view is automatically refreshed whenever a commit operation occurs on the underlying base tables. This method is one of the automatic refresh strategies available in Oracle, and it ensures that the materialized view stays synchronized with its base tables in near real-time, reflecting any changes made via INSERT, UPDATE, or DELETE statements that are committed.

 

1. What is On Commit Refresh?

On Commit Refresh means the materialized view is refreshed immediately after a commit to the base table(s) occurs. In other words, when data changes in the base tables and a commit is executed, Oracle automatically refreshes the materialized view to reflect those changes.

• Commit in Oracle refers to the action of saving changes made during a transaction to the database permanently.
• When On Commit Refresh is configured for a materialized view, the materialized view is refreshed automatically right after the base table data is committed.

 

2. How Does On Commit Refresh Work?

When you create a materialized view with an On Commit Refresh, Oracle triggers the refresh of the materialized view after a commit operation to any of the base tables that the materialized view depends on.

Key Points:

• The refresh process is automatic, so the materialized view is kept up-to-date without requiring manual intervention.
• The refresh happens only after a commit to the base table data, ensuring that the materialized view reflects committed data.
• It is useful when you want the materialized view to remain current without needing a scheduled or manual refresh but still want to avoid refreshing on every minor change.

 

3. When Should I Use On Commit Refresh?

You should consider using On Commit Refresh in the following scenarios:

• Real-time synchronization: When you need the materialized view to reflect the most recent data immediately after a commit, and you don't want to wait for a scheduled refresh.
• Small or incremental data changes: When data changes in the base table happen in small increments, and you want to refresh the materialized view after each commit.
• Transactional consistency: When you need the materialized view to be in sync with the database after each transaction commit, ensuring that reports or queries using the materialized view always work with committed data.

 

4. How to Configure On Commit Refresh?

To configure a materialized view to refresh on commit, you specify the ON COMMIT clause when creating the materialized view.

Here is an example:

CREATE MATERIALIZED VIEW mv_name

REFRESH ON COMMIT

AS

SELECT column1, column2

FROM base_table;

• The REFRESH ON COMMIT clause specifies that the materialized view will be automatically refreshed after any commit to the base_table or any other base tables that the view depends on.

 

5. Advantages of On Commit Refresh

• Automatic Synchronization: The materialized view is kept up-to-date automatically after each commit, without requiring additional user intervention or complex scheduling.
• Real-Time Data Reflection: The materialized view always reflects the latest committed data from the base tables, ensuring that reports or queries are run against the most current data.
• No Need for Scheduling: You don't need to set up complex schedules for refreshing the materialized view, as the refresh happens automatically with every commit.
• Efficient for Low Volume Updates: For environments where data changes are incremental or low volume, this refresh method ensures that the materialized view remains accurate without needing a complete refresh.

 

6. Disadvantages of On Commit Refresh

• Performance Overhead: Since the refresh occurs immediately after a commit, it could cause additional overhead, especially in systems with high transaction rates. Each commit will trigger a refresh, potentially impacting performance.
• Increased Locking: Refreshing the materialized view after each commit could cause locking issues on the base tables or materialized view, potentially impacting concurrent access.
• Limited to Base Table Changes: If a materialized view depends on multiple base tables and not all of them are updated, the view still refreshes upon any commit, which may be unnecessary in some cases.
• May Not Scale for Large Datasets: For large materialized views or systems with heavy updates, On Commit Refresh may become inefficient because it could trigger frequent and potentially expensive refresh operations.

 

7. When Should I Avoid On Commit Refresh?

Avoid using On Commit Refresh in the following cases:

• High Transaction Volume: In environments with a high frequency of transactions and commits, this refresh method could create significant performance issues due to constant refresh operations.
• Large Materialized Views: For large materialized views with complex queries, On Commit Refresh could cause performance degradation as each commit triggers a refresh, potentially making the refresh process expensive.
• Unnecessary Refreshes: If only a small portion of the data in the base tables changes and doesn't affect the materialized view, a commit will still trigger the refresh, which might not be needed.

 

8. Refresh Process for On Commit

The refresh process when using On Commit Refresh follows these steps:

1. Transaction Commits: A transaction modifies data in the base table(s).
2. Commit Action: The user commits the transaction, making changes to the database permanent.
3. Materialized View Refresh: Upon commit, Oracle automatically triggers a refresh for the materialized view. This refresh could be a Fast Refresh or a Complete Refresh, depending on the configuration and whether the materialized view is capable of a fast refresh (such as when materialized view logs are present).

 

9. Example of On Commit Refresh with Fast Refresh

When using On Commit Refresh, if the materialized view is designed to use Fast Refresh (typically supported for materialized views with logs), Oracle will apply only the incremental changes from the base tables to the materialized view after the commit.

CREATE MATERIALIZED VIEW mv_name

BUILD IMMEDIATE

REFRESH ON COMMIT

AS

SELECT column1, column2

FROM base_table;

In this case, Fast Refresh would occur if a materialized view log exists on the base table. Otherwise, Oracle would default to a Complete Refresh.

 

10. How to Monitor On Commit Refresh?

You can monitor the status of On Commit Refresh by checking the refresh times and methods using the DBA_MVIEW_REFRESH_TIMES view. For example:

SELECT * FROM DBA_MVIEW_REFRESH_TIMES

WHERE mview_name = 'YOUR_MVIEW_NAME';

This will show the timestamp and refresh method used for the materialized view.

Additionally, if you are experiencing performance issues, checking for frequent refreshes might help identify whether On Commit Refresh is causing excessive refresh activity.

 

11. Best Practices for On Commit Refresh

• Use for Low-Volume Data Changes: On Commit Refresh works best when base tables are updated in small amounts or on a less frequent basis. In high-throughput systems, other refresh strategies (like On Demand or Scheduled Refresh) might be better.
• Optimize Materialized View Logs: Ensure that materialized view logs are configured on the base tables, as this helps speed up Fast Refresh operations, reducing the performance overhead during commit-based refreshes.
• Balance Between Performance and Freshness: Carefully assess whether the need for real-time data consistency outweighs the performance impact of frequent refreshes. In systems with high commit rates, it may be better to rely on Scheduled Refresh.

 

12. Conclusion

On Commit Refresh is a powerful option for ensuring that materialized views are updated immediately following data changes. It’s ideal when you require near real-time synchronization of materialized views with base tables and can tolerate the overhead associated with frequent refreshes. However, it may not be suitable for systems with high transaction volumes or large datasets, where performance optimization is critical.

By understanding the advantages, disadvantages, and best practices for On Commit Refresh, you can make informed decisions on whether this refresh method is appropriate for your specific requirements.