A Complete Refresh involves re-executing the query that defines the materialized view. This process completely rebuilds the materialized view from scratch by retrieving all the data again from the base tables. It ensures that the materialized view is fully synchronized with the current state of the underlying data.
2. When should I use Complete Refresh instead of Fast Refresh?
You should use Complete Refresh when:
- The materialized view is based on complex queries, such as those with joins, aggregations, or subqueries.
- The base tables do not have primary or unique keys, which are required for fast refresh.
- The materialized view requires full recalculation of the data, and incremental updates are not feasible.
- You need to ensure the materialized view is fully updated after changes, especially if fast refresh has failed.
3. How does Complete Refresh work in Oracle?
In a Complete Refresh, the materialized view is completely rebuilt by re-running the query. It fetches the entire dataset from the base tables, replacing the old data in the materialized view with the new data. No materialized view log is required because it doesn't rely on incremental changes.
4. How do I perform a Complete Refresh on a materialized view?
You can perform a complete refresh using the DBMS_MVIEW.REFRESH procedure. Example:
EXEC DBMS_MVIEW.REFRESH('sales_mv', 'C');
Where:
- 'sales_mv' is the name of the materialized view.
- 'C' specifies that it is a Complete Refresh.
Alternatively, you can use the Force Refresh ('R'), which tries a fast refresh first and falls back to a complete refresh if fast refresh is not possible.
5. What are the disadvantages of using Complete Refresh?
- High Resource Usage: Since it re-executes the entire query and recalculates all data, a complete refresh consumes more CPU, I/O, and memory resources than a fast refresh.
- Longer Refresh Times: For large datasets or complex queries, a complete refresh can take significantly longer to complete than a fast refresh.
- Not Efficient for Real-Time Needs: If you need the materialized view to reflect real-time changes, a complete refresh is not ideal since it cannot be performed as frequently as a fast refresh.
6. Is Complete Refresh required for all materialized views?
No, Complete Refresh is not required for all materialized views. It is typically used when fast refresh is not feasible due to query complexity, lack of primary keys in base tables, or when the materialized view cannot be incrementally refreshed.
7. Can I schedule a Complete Refresh in Oracle?
Yes, you can schedule a Complete Refresh using Oracle's DBMS_SCHEDULER or DBMS_JOB to run at specific times. This is particularly useful for large materialized views, as you may want to schedule the refresh during off-peak hours to minimize system load.
Example:
BEGIN
DBMS_SCHEDULER.create_job (
job_name => 'refresh_sales_mv',
job_type => 'PLSQL_BLOCK',
job_action => 'BEGIN DBMS_MVIEW.REFRESH(''sales_mv'', ''C''); END;',
start_date => SYSTIMESTAMP,
repeat_interval => 'FREQ=DAILY; BYHOUR=2; BYMINUTE=0;',
enabled => TRUE
);
END;
/
This example schedules a complete refresh for sales_mv every day at 2:00 AM.
8. Can I use Complete Refresh for large materialized views?
Yes, Complete Refresh can be used for large materialized views, but it may take longer to complete and can consume significant resources. You may want to optimize the performance of the refresh process by indexing the materialized view and its underlying tables or by scheduling the refresh during off-peak times.
9. Can I monitor the status of a Complete Refresh?
Yes, you can monitor the status of a complete refresh using views like DBA_MVIEW_REFRESH_TIMES or USER_MVIEW_REFRESH_TIMES. These views store information about the refresh times, types, and statuses.
Example:
SELECT * FROM DBA_MVIEW_REFRESH_TIMES
WHERE mview_name = 'SALES_MV';
This will show when the last refresh took place and whether it was complete or fast.
10. What are the key benefits of using Complete Refresh?
- Simplicity: It is a simple method of ensuring the materialized view is fully synchronized with the base tables, regardless of the complexity of the underlying query.
- Data Consistency: Since it recalculates the entire materialized view, it ensures the data is consistent and up-to-date.
- No Materialized View Log Required: Unlike fast refresh, which requires a materialized view log, complete refresh does not need any special tracking of changes to the base tables.
11. Can I use Complete Refresh with Force Refresh?
Yes, you can use Force Refresh ('R'), which first attempts a fast refresh and falls back to a complete refresh if fast refresh is not possible. This approach helps optimize performance by using fast refresh when it is feasible.
Example:
EXEC DBMS_MVIEW.REFRESH('sales_mv', 'R');
12. How do I handle issues during Complete Refresh?
If issues arise during a complete refresh (such as performance degradation), consider the following steps:
- Review Indexing: Ensure that both the materialized view and the base tables are well-indexed to optimize the query execution.
- Optimize Query: Look for opportunities to optimize the underlying query, such as reducing unnecessary joins or aggregations.
- Schedule During Off-Peak Hours: If refresh time is long, schedule it during off-peak hours to minimize the impact on system performance.
- Check System Resources: Monitor CPU, memory, and disk usage during refreshes and ensure the system has sufficient resources.
13. What should I do if a Complete Refresh fails?
- Check Query: Verify that the query defining the materialized view is valid and does not have errors.
- System Resources: Ensure that the system has enough resources (e.g., CPU, memory) to complete the refresh process.
- Error Logs: Check for errors in the Oracle alert log or materialized view refresh logs for more details about what went wrong.
- Retry the Refresh: Sometimes, retrying the refresh after resolving any issues can resolve the problem.
14. Can Complete Refresh be automated?
Yes, you can automate Complete Refresh using DBMS_SCHEDULER or DBMS_JOB to schedule the refresh at regular intervals. This is particularly useful for large materialized views that need periodic updates.
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