Force Refresh FAQS

1. What is a Force Refresh in Oracle?

A Force Refresh in Oracle is a refresh strategy for materialized views that first attempts to perform a Fast Refresh and, if that fails (due to missing materialized view logs or complex queries), automatically falls back to a Complete Refresh. This ensures that the materialized view is refreshed, even if the more efficient Fast Refresh cannot be executed.

 

2. When should I use Force Refresh?

You should use Force Refresh when:

• You want Oracle to automatically decide between a Fast Refresh or Complete Refresh, depending on the situation.
• You have complex queries or missing materialized view logs that prevent fast refresh from working.
• You need to ensure that the materialized view is always updated, regardless of the feasibility of a fast refresh.

 

3. How does Force Refresh work?

• Step 1: Oracle checks if a Fast Refresh can be done (based on materialized view logs and the query).
• Step 2: If a Fast Refresh is possible, Oracle applies only the changes (inserts, updates, deletes).
• Step 3: If Fast Refresh is not possible, Oracle falls back to a Complete Refresh, which rebuilds the entire materialized view from scratch.

 

4. How do I perform a Force Refresh?

You can perform a Force Refresh using the following SQL command:

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

Where:

• 'materialized_view_name' is the name of the materialized view.
• 'R' specifies a Force Refresh.

 

5. What is the difference between Force Refresh and Fast Refresh?

• Fast Refresh only applies incremental changes (inserts, updates, deletes) to the materialized view. It requires materialized view logs and is faster.
• Force Refresh tries a Fast Refresh first. If that’s not possible, it will fall back to a Complete Refresh, which rebuilds the materialized view from scratch.

 

6. Why would Fast Refresh fail and Force Refresh be used?

Fast Refresh can fail due to:

• Missing materialized view logs on the base tables.
• The query being too complex or involving operations that cannot be incrementally refreshed (e.g., complex joins, aggregation).
• Changes to the structure of base tables (e.g., missing keys, foreign key relationships).

If Fast Refresh fails, Force Refresh will perform a Complete Refresh.

 

7. What are the advantages of using Force Refresh?

• Flexibility: It ensures the materialized view is updated by first attempting a Fast Refresh and falling back to a Complete Refresh when necessary.
• Consistency: Guarantees that the materialized view is refreshed, even if the fast refresh is not feasible.
• Reduced Manual Effort: Reduces the need to manually decide between fast or complete refreshes, as Oracle handles it automatically.

 

8. What are the disadvantages of Force Refresh?

• Performance: If Force Refresh falls back to Complete Refresh, it can be resource-intensive, especially for large materialized views, as it rebuilds the entire view.
• Longer Refresh Time: The Complete Refresh process is generally slower and more resource-consuming than the Fast Refresh.
• Possible Unnecessary Refreshes: If the base tables have minimal changes, the Complete Refresh might still be performed unnecessarily, using more resources than required.

 

9. Can Force Refresh be scheduled?

Yes, you can schedule a Force Refresh using DBMS_SCHEDULER or DBMS_JOB. For example, you can schedule a Force Refresh to occur during off-peak hours to reduce the impact on system performance.

Example using DBMS_SCHEDULER:

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;

/

 

10. What is the difference between Force Refresh and Complete Refresh?

• Complete Refresh always rebuilds the materialized view from scratch, regardless of whether incremental changes are possible.
• Force Refresh tries a Fast Refresh first and will only perform a Complete Refresh if Fast Refresh is not possible.

 

11. Can I monitor the status of Force Refresh?

Yes, you can monitor the status of a refresh operation using views like DBA_MVIEW_REFRESH_TIMES or USER_MVIEW_REFRESH_TIMES. These views provide information about the last refresh time, type of refresh (fast, complete, or force), and status.

Example:

SELECT * FROM DBA_MVIEW_REFRESH_TIMES

WHERE mview_name = 'SALES_MV';

 

12. Can I use Force Refresh with multiple materialized views at once?

Yes, you can refresh multiple materialized views in one execution by providing their names in an array or calling DBMS_MVIEW.REFRESH multiple times in a script.

 

13. How do I troubleshoot Force Refresh failures?

If Force Refresh fails:

• Check Query Validity: Ensure the query defining the materialized view is correct and valid.
• Check Materialized View Logs: Verify that materialized view logs are present and updated for the base tables.
• Check Resource Usage: Monitor the system’s CPU, memory, and I/O during refresh to ensure it has enough resources.
• Review Errors: Check Oracle's alert logs or error logs for detailed messages regarding the refresh failure.

 

14. Can Force Refresh be used with all types of materialized views?

Force Refresh can be used with most types of materialized views, but it requires certain conditions (e.g., materialized view logs) for a Fast Refresh to be successful. If these conditions are not met, Oracle will fall back to a Complete Refresh.

 

15. Is Force Refresh recommended for large materialized views?

Using Force Refresh on large materialized views may result in significant resource consumption if the fallback to Complete Refresh is required. It's advisable to carefully monitor the refresh times and consider scheduling refreshes during off-peak hours to mitigate performance issues.