Force Refresh

A Force Refresh in Oracle is a refresh method for materialized views that allows the database to first attempt a Fast Refresh and, if that is not possible (due to certain conditions), automatically fall back to a Complete Refresh. This hybrid refresh strategy provides flexibility and ensures that materialized views are updated, even when a fast refresh cannot be performed.

 

1. What is Force Refresh?

A Force Refresh is a mechanism that:

  • First tries a Fast Refresh: Oracle attempts to perform a Fast Refresh by applying incremental changes (inserts, updates, deletes) to the materialized view.
  • Falls back to a Complete Refresh: If Fast Refresh cannot be done (for example, if there are no materialized view logs or the query is too complex), Oracle will fall back to performing a Complete Refresh. This rebuilds the entire materialized view from scratch by re-running the query that defines the view.

 

2. When to Use Force Refresh?

You would use Force Refresh in situations where:

  • You want to optimize refresh performance: Oracle will try to use the faster Fast Refresh method when possible but ensure that the refresh still happens, even if a fast refresh isn't possible.
  • The materialized view query is complex: For complex queries that might not support a Fast Refresh, using Force Refresh ensures the materialized view is refreshed regardless of the query complexity.
  • You’re uncertain about the ability to perform Fast Refresh: In cases where you want to ensure that the materialized view will be refreshed even if the Fast Refresh fails (e.g., missing materialized view logs or issues with the base tables).

 

3. How to Perform a Force Refresh

You can initiate a Force Refresh using the DBMS_MVIEW.REFRESH procedure. This will attempt a Fast Refresh first and fall back to Complete Refresh if needed.

Example:

EXEC DBMS_MVIEW.REFRESH('sales_mv', 'R');

Where:

  • 'sales_mv' is the name of the materialized view.
  • 'R' stands for Force Refresh.

The Force Refresh method allows Oracle to first try a fast refresh, and if that is not possible, it will perform a complete refresh.

 

4. How Does Force Refresh Work?

  • Step 1: Oracle checks whether a Fast Refresh is possible based on the presence of materialized view logs and the query's complexity.
  • Step 2: If a Fast Refresh is possible (e.g., all changes can be tracked and applied incrementally), Oracle applies only the changes to the materialized view.
  • Step 3: If a Fast Refresh is not possible (due to complexity, lack of materialized view logs, or base table issues), Oracle will automatically fall back to a Complete Refresh. This rebuilds the entire materialized view from scratch.

 

5. When Is Force Refresh Useful?

  • Unknown Query Complexity: If you're unsure whether the materialized view query can support a fast refresh, using Force Refresh ensures that the refresh will still happen, either by fast or complete refresh.
  • Changing Database Structure: When the database schema or underlying tables change (e.g., adding new columns or indexes), it may disrupt the fast refresh process. A Force Refresh allows Oracle to adapt and choose the appropriate method.
  • Ensuring Materialized View Consistency: If you need to guarantee that the materialized view is refreshed, regardless of the possibility of fast refresh, Force Refresh ensures the view will always be updated.

 

6. Example of Force Refresh Usage

Suppose you have a materialized view called sales_mv and want to perform a Force Refresh to ensure it's updated with the most current data:

EXEC DBMS_MVIEW.REFRESH('sales_mv', 'R');

This will:

  • Try to perform a Fast Refresh (if possible).
  • If the fast refresh cannot be performed (due to missing logs or complex query), it will automatically fall back to a Complete Refresh.

 

7. Advantages of Force Refresh

  • Flexibility: Force Refresh allows Oracle to try the more efficient Fast Refresh first and only fall back to Complete Refresh when necessary, ensuring both performance and data accuracy.
  • Automatic Failover: The ability to automatically fall back to a complete refresh removes the need for manual intervention if a fast refresh is not possible.
  • Consistency: It ensures that the materialized view is refreshed, regardless of issues with fast refresh compatibility.
  • Performance Optimization: When possible, the Fast Refresh method is used, minimizing resource consumption and reducing the time it takes to update the materialized view.

 

8. Limitations of Force Refresh

  • Performance Impact if Falling Back to Complete Refresh: While Force Refresh tries to optimize the process by using Fast Refresh when possible, if the system falls back to a Complete Refresh, it may result in higher resource usage and longer refresh times, especially for large materialized views.
  • Complex Query Handling: If the materialized view query is too complex (e.g., involving non-supported operations like certain types of joins, aggregates, or subqueries), the fallback to complete refresh may still be resource-intensive.

 

9. Materialized View Log for Force Refresh

For Force Refresh to succeed with Fast Refresh, the following conditions should be met:

  • Materialized View Log: A materialized view log must exist on the base tables. This log tracks changes made to the base table (insertions, deletions, and updates) and is required for fast refresh to be performed efficiently.
  • Primary/Unique Key: The base table should ideally have a primary or unique key for fast refresh to work.

Without these, Oracle will fall back to performing a Complete Refresh, even if Force Refresh is requested.

 

10. Monitoring Force Refresh

You can monitor the refresh status and time using the DBA_MVIEW_REFRESH_TIMES view, which provides information on the most recent refresh attempts and whether the refresh was fast or complete.

Example query:

SELECT * FROM DBA_MVIEW_REFRESH_TIMES

WHERE mview_name = 'SALES_MV';

This will show when the last refresh took place and whether it was fast or complete, helping you understand how the Force Refresh operation was handled.

 

11. Force Refresh and Scheduling

Just like other types of refresh, you can schedule Force Refresh to run automatically at regular intervals using DBMS_SCHEDULER or DBMS_JOB. For example, to refresh a materialized view every day at midnight:

BEGIN

   DBMS_SCHEDULER.create_job (

      job_name        => 'refresh_sales_mv',

      job_type        => 'PLSQL_BLOCK',

      job_action      => 'BEGIN DBMS_MVIEW.REFRESH(''sales_mv'', ''R''); END;',

      start_date      => SYSTIMESTAMP,

      repeat_interval => 'FREQ=DAILY; BYHOUR=0; BYMINUTE=0;',

      enabled         => TRUE

   );

END;

/

 

12. Best Practices for Force Refresh

  • Monitor Materialized View Logs: Ensure that the materialized view logs are properly maintained and contain the necessary data to support a fast refresh when possible.
  • Use Scheduling for Efficiency: For large materialized views, schedule Force Refresh during off-peak hours to avoid performance issues during business hours.
  • Use Force Refresh Sparingly: While Force Refresh provides flexibility, frequent reliance on Complete Refresh may negate the benefits of fast refresh, particularly in large datasets. Aim for a balance between performance and data consistency.

 

Summary

  • Force Refresh provides a flexible mechanism that first attempts a Fast Refresh and falls back to a Complete Refresh if necessary.
  • It ensures that materialized views are updated, even if fast refresh cannot be performed due to query complexity or missing materialized view logs.
  • Force Refresh can help automate and optimize refreshes for materialized views without requiring manual intervention, but should be used carefully to avoid unnecessary resource usage, particularly if it falls back to a complete refresh.

If you need further clarification or have more questions, feel free to ask!

 

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