IMMEDIATE Refresh

In Oracle, a materialized view is a database object that stores the results of a query physically on disk. It is similar to a regular view, but unlike a view that dynamically computes its result set each time it is queried, a materialized view stores the result set persistently and can be refreshed periodically to keep the data up to date.

The IMMEDIATE refresh option is used when creating or refreshing a materialized view. It indicates that the view's data should be refreshed immediately after the materialized view is created or when a manual refresh is triggered.

Let’s dive deeper into the details of IMMEDIATE refresh for materialized views in Oracle.

1. What is a Materialized View in Oracle?

A materialized view is a view whose data is physically stored on disk. It is created using a query that defines the view, and the result of that query is materialized (or persisted). The data can be refreshed periodically to ensure it stays current.

2. What is IMMEDIATE Refresh for Materialized Views?

The IMMEDIATE refresh option specifies that the materialized view should be refreshed immediately after it is created or explicitly refreshed.

• Immediate Refresh: This means that the data of the materialized view will be instantly refreshed when the CREATE or REFRESH command is executed. It fetches the latest data from the underlying base tables and stores it in the materialized view.
• This type of refresh is typically used when you want the materialized view to reflect the most current data at the time of creation or refresh, without waiting for any delay.

In contrast, there are other refresh strategies like ON COMMIT and ON DEMAND that behave differently when it comes to when the refresh occurs.

3. Syntax for Creating a Materialized View with IMMEDIATE Refresh

When creating a materialized view, you can specify the IMMEDIATE refresh method using the REFRESH clause. Here is the syntax:

CREATE MATERIALIZED VIEW view_name

REFRESH IMMEDIATE

START WITH sysdate

NEXT sysdate + 1/24

AS

SELECT column1, column2, ...

FROM table_name

WHERE condition;

• REFRESH IMMEDIATE: Ensures that the materialized view’s data is refreshed immediately after it is created or refreshed manually.
• START WITH sysdate: Specifies when the refresh should start. In this case, it is set to the current date and time.
• NEXT sysdate + 1/24: Specifies when the next refresh will occur. Here, the view will refresh every hour (1/24 of a day).

4. How Does the IMMEDIATE Refresh Work?

When you create or refresh a materialized view with IMMEDIATE, the following process occurs:

1. Initial Population: If the materialized view is being created, Oracle will immediately fetch the data from the underlying tables (or query) and store it in the materialized view.
2. Subsequent Refreshes: If the materialized view already exists, an IMMEDIATE refresh will update the data stored in the materialized view by querying the base tables again, ensuring it holds the most up-to-date data.
• The refresh occurs immediately after the REFRESH command is executed.
• This ensures that users who query the materialized view will always see the most current data.

5. Performance Considerations of IMMEDIATE Refresh

Using IMMEDIATE refresh has certain implications on performance:

• Resource-Intensive: Since the data is refreshed immediately, this operation can be resource-intensive, especially if the underlying query involves large tables or complex joins. It could impact system performance while the refresh is in progress.
• Locks: During the refresh, Oracle may place locks on the materialized view and/or underlying tables to maintain data consistency. These locks could affect other processes that need to access the same resources.
• Impact on Base Tables: The refresh process may involve querying large amounts of data from the base tables, which can increase the load on the database.
• Concurrency: Refreshing the materialized view immediately may affect the performance of other queries that are querying the same data at the same time.

Therefore, IMMEDIATE refresh is typically used when it is essential to have the most up-to-date data in the materialized view at all times, but you should weigh the performance trade-offs carefully.

6. Examples of IMMEDIATE Refresh for Materialized Views

Example 1: Creating a Materialized View with IMMEDIATE Refresh

Here’s an example where a materialized view is created with an immediate refresh interval:

CREATE MATERIALIZED VIEW sales_mv

REFRESH IMMEDIATE

START WITH SYSDATE

NEXT SYSDATE + 1/24 -- Refresh every hour

AS

SELECT product_id, SUM(sales_amount) AS total_sales

FROM sales

GROUP BY product_id;

In this example:

• The materialized view sales_mv will be created with the result of the query.
• The data will be refreshed immediately after the view is created.
• After the initial refresh, the materialized view will be refreshed every hour.

Example 2: Manually Refreshing a Materialized View Immediately

If a materialized view already exists, you can manually refresh it immediately using the following command:

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

This will immediately refresh the sales_mv materialized view with the latest data from the underlying base table (sales in this case).

7. Refresh Strategies: IMMEDIATE vs. ON COMMIT vs. ON DEMAND

• IMMEDIATE: Refreshes the materialized view immediately when the refresh is triggered, ensuring the most up-to-date data is available. However, it can be resource-intensive, as discussed.
• ON COMMIT: Refreshes the materialized view automatically when a transaction is committed to the base tables. This is useful when you want the materialized view to stay in sync with the transactional data, but it may be slower for large datasets or systems with high transaction rates.
• ON DEMAND: Allows manual refreshing of the materialized view whenever needed. You control when the view is refreshed, offering flexibility but not real-time data updates.

8. Benefits of Using IMMEDIATE Refresh for Materialized Views

• Real-Time Data: The IMMEDIATE refresh ensures that the materialized view holds the most current data from the underlying base tables. This is important when working with critical data that needs to reflect the most up-to-date information.
• Consistency: By ensuring immediate refresh, users querying the materialized view will always get consistent and current results, without needing to worry about outdated data.
• Query Performance: Queries that use materialized views with IMMEDIATE refresh can be faster because the data is precomputed and stored, avoiding the need for repeated computation each time the view is queried.

9. Use Cases for IMMEDIATE Refresh

• Reporting: If your application requires up-to-the-minute reports, such as financial summaries or inventory counts, a materialized view with IMMEDIATE refresh can help you provide real-time or near-real-time reporting.
• Data Warehousing: In data warehousing environments, materialized views can aggregate large amounts of data from multiple sources. If it's crucial to have the latest aggregated data, using an IMMEDIATE refresh ensures the data is always up to date.
• Caching: Materialized views can act as a cache for frequently queried data. By refreshing them immediately, you ensure that users querying the cache get the freshest data without the need for expensive joins or calculations.

10. Limitations of IMMEDIATE Refresh

• Performance Overhead: IMMEDIATE refresh can be expensive in terms of time and resources, especially if the materialized view involves large tables or complex queries.
• Locking: The refresh operation may involve locking the materialized view and/or base tables, which can impact concurrent users or applications.
• Cannot be Scheduled: While you can specify the start time and refresh interval, the refresh is always triggered immediately when the view is created or manually refreshed, so it cannot be delayed.

11. Dropping a Materialized View with IMMEDIATE Refresh

You can drop a materialized view with the following command:

DROP MATERIALIZED VIEW sales_mv;

This will remove the materialized view, including its stored data, from the database.

Conclusion

The IMMEDIATE refresh option for materialized views in Oracle ensures that the data in the view is always up to date immediately after the materialized view is created or refreshed. This is ideal for scenarios where real-time or near-real-time data is essential, such as reporting, data warehousing, or caching. However, due to the potential performance impact, especially for large datasets, careful consideration should be given to whether IMMEDIATE refresh is the right choice based on the system's performance requirements and workload.