1. What is a DEFERRED refresh in Oracle?
A DEFERRED refresh refers to the delay in refreshing a materialized view. Instead of refreshing automatically after every change in the underlying data, the refresh happens at a later, explicitly defined time, typically triggered manually or by a scheduled job.
2. Why should I use a DEFERRED refresh for materialized views?
- Performance Optimization: If the underlying data changes frequently, but real-time consistency isn’t critical, deferring the refresh helps reduce the performance overhead by avoiding constant refreshes.
- Batch Processing: It allows you to refresh data during off-peak hours (e.g., nightly), which is useful when working with large datasets or complex queries.
- Reduced Locking: Immediate refreshes can cause locks on tables or materialized views. Deferred refresh reduces the impact on these operations.
3. How do I implement a DEFERRED refresh in Oracle?
You can specify DEFERRED in the CREATE MATERIALIZED VIEW or ALTER MATERIALIZED VIEW statement, like this:
CREATE MATERIALIZED VIEW sales_mv
BUILD IMMEDIATE
REFRESH DEFERRED
AS
SELECT * FROM sales;
This ensures that the refresh is postponed until explicitly triggered.
4. How do I manually refresh a materialized view with DEFERRED refresh?
You can refresh the materialized view using the DBMS_MVIEW.REFRESH procedure, as shown below:
EXEC DBMS_MVIEW.REFRESH('sales_mv');
You can also specify the refresh type (e.g., FAST, COMPLETE, FORCE).
5. Can I automate the DEFERRED refresh?
Yes, you can schedule the refresh using DBMS_SCHEDULER. For example, to schedule a daily refresh at 2:00 AM, use the following:
BEGIN
DBMS_SCHEDULER.create_job (
job_name => 'refresh_sales_mv',
job_type => 'PLSQL_BLOCK',
job_action => 'BEGIN DBMS_MVIEW.REFRESH(''sales_mv''); END;',
start_date => SYSTIMESTAMP,
repeat_interval => 'FREQ=DAILY; BYHOUR=2; BYMINUTE=0;',
enabled => TRUE
);
END;
/
6. What are the risks of using a DEFERRED refresh?
- Data Staleness: The materialized view might show outdated data if the refresh is not done regularly, which can be problematic for real-time reporting or analytics.
- Manual Control: Since the refresh is manual, there's a risk that you may forget to refresh the materialized view or delay it too long.
- Complexity: Managing schedules for refresh can become complex, especially if there are multiple materialized views or interdependencies.
7. How can I check the last refresh time of a materialized view?
You can use the following query to check when a materialized view was last refreshed:
SELECT * FROM DBA_MVIEW_REFRESH_TIMES
WHERE mview_name = 'SALES_MV';
This query will show the timestamp of the last refresh for the specified materialized view.
8. What’s the difference between DEFERRED and IMMEDIATE refresh?
- DEFERRED refresh is a delayed refresh that occurs manually or via a scheduled job. It allows more control over when the refresh happens.
- IMMEDIATE refresh occurs automatically as soon as the base data is modified, which can be useful for real-time updates but can lead to higher system load.
9. Can I specify a specific refresh method (FAST, COMPLETE) with DEFERRED refresh?
Yes, when triggering a refresh manually, you can specify the refresh method like FAST, COMPLETE, or FORCE. For example:
EXEC DBMS_MVIEW.REFRESH('sales_mv', 'C'); -- Complete refresh
Where:
- 'C' stands for Complete.
- 'F' stands for Fast.
- 'R' stands for Force.
10. How do I handle large datasets with DEFERRED refresh?
For large datasets, performing a complete refresh might take significant time and resources. In such cases, consider:
- Fast Refresh (if possible): This refreshes only the changes to the materialized view rather than the whole dataset.
- Scheduling the Refresh: Schedule the refresh during low-traffic times (e.g., overnight) to minimize the impact on the system.
If you have any more questions or need further assistance, feel free to ask!
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