Fast Refresh FAQS

1. What is Fast Refresh in Oracle?

Fast Refresh is a method to refresh a materialized view by applying only the changes made to the underlying base tables since the last refresh. This allows for a more efficient update, avoiding the need to re-run the entire query that defines the materialized view, making it faster and less resource-intensive.

 

2. What conditions must be met for a materialized view to support Fast Refresh?

For a materialized view to support Fast Refresh, the following conditions must be met:

  • Primary or Unique Key: The base table must have a primary key or a unique key.
  • Materialized View Log: A materialized view log (snapshot log) must be created on the base table to record changes (inserts, updates, deletes).
  • Simple Query: The materialized view should typically be based on simple queries, usually selecting from a single table, although it can support certain complex queries with additional configurations.

 

3. What is a Materialized View Log?

A Materialized View Log is a special table that records changes made to the base table (insert, update, delete operations). The log allows Oracle to track changes and apply them efficiently during the fast refresh process.

 

4. How do I create a Materialized View Log?

To create a materialized view log, use the CREATE MATERIALIZED VIEW LOG command:

CREATE MATERIALIZED VIEW LOG ON sales

WITH ROWID, PRIMARY KEY, SEQUENCE;

This log tracks changes made to the sales table, including updates to the primary key, rowid, and sequence number.

 

5. How do I perform a Fast Refresh on a materialized view?

To manually perform a fast refresh, use the DBMS_MVIEW.REFRESH procedure. For example:

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

Where:

  • 'sales_mv' is the name of the materialized view.
  • 'F' specifies that it is a Fast Refresh.

 

6. What happens if a Fast Refresh is not possible?

If Oracle cannot perform a fast refresh due to the complexity of the materialized view (e.g., if the query involves joins, aggregations, or lacks the necessary materialized view log), it will fall back to a Complete Refresh, which rebuilds the entire materialized view.

 

7. Can I automate Fast Refresh?

Yes, you can schedule the fast refresh using Oracle's DBMS_SCHEDULER. For example, you can set it to refresh at specific intervals:

BEGIN

   DBMS_SCHEDULER.create_job (

      job_name        => 'refresh_sales_mv',

      job_type        => 'PLSQL_BLOCK',

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

      start_date      => SYSTIMESTAMP,

      repeat_interval => 'FREQ=DAILY; BYHOUR=2; BYMINUTE=0;',

      enabled         => TRUE

   );

END;

/

This example schedules a fast refresh for sales_mv every day at 2:00 AM.

 

8. What are the benefits of using Fast Refresh?

  • Performance: Fast refresh reduces the system load by updating only the changed data rather than recalculating the entire materialized view.
  • Efficiency: For large datasets, performing a fast refresh can be much faster than a complete refresh.
  • Minimized Resource Usage: Only incremental changes are applied, reducing the amount of resources (CPU, I/O, etc.) required for the refresh process.

 

9. What are the limitations of Fast Refresh?

  • Data Complexity: If the materialized view involves complex queries such as joins, aggregations, or subqueries, fast refresh may not be supported without modifications.
  • Materialized View Log Maintenance: The materialized view log needs to be maintained, which introduces additional overhead on the system.
  • Changes to Non-Indexed Columns: If changes are made to columns that are not part of the primary or unique key, the fast refresh may not work efficiently.

 

10. How can I monitor the status of a Fast Refresh?

You can monitor the status and history of materialized view refreshes using the DBA_MVIEW_REFRESH_TIMES or USER_MVIEW_REFRESH_TIMES views. These views provide information about the last refresh time, type of refresh, and other relevant details.

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 fast or complete.

 

11. What is the difference between Fast Refresh and Complete Refresh?

  • Fast Refresh updates only the changes made to the base tables since the last refresh. It is faster and uses fewer resources.
  • Complete Refresh rebuilds the entire materialized view by re-running the query, which is more resource-intensive and slower, especially for large datasets.

 

12. Can I specify different refresh methods (Fast, Complete, Force)?

Yes, you can specify the refresh method when manually refreshing a materialized view:

  • 'F': Fast Refresh
  • 'C': Complete Refresh
  • 'R': Force Refresh (Oracle will attempt Fast Refresh first, and if it fails, fall back to Complete Refresh)

For example:

EXEC DBMS_MVIEW.REFRESH('sales_mv', 'F');  -- Fast Refresh

EXEC DBMS_MVIEW.REFRESH('sales_mv', 'C');  -- Complete Refresh

EXEC DBMS_MVIEW.REFRESH('sales_mv', 'R');  -- Force Refresh

 

13. What should I do if Fast Refresh is failing?

  • Check Materialized View Log: Ensure that the materialized view log exists and contains the necessary information (e.g., primary key, rowid).
  • Verify Query Simplicity: Ensure the query used in the materialized view is simple enough to support fast refresh (e.g., avoid complex joins or subqueries).
  • Check for Updates to Primary Key: Fast refresh relies on changes to primary keys or unique keys, so ensure these are being tracked correctly.

 

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

 

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