In Oracle, views and materialized views are two types of database objects that allow you to simplify complex queries and improve data access. While they share some similarities, there are significant differences between them.
Introduction to Views
A view is a virtual table based on the result-set of a query. It does not store data itself, but instead, it references the underlying tables to retrieve data when queried. When you create a view, Oracle stores the query definition, and every time you access the view, the database executes the query to fetch the latest data.
Views are useful for several reasons:
- They simplify complex queries by breaking them down into smaller, more manageable pieces.
- They provide a layer of abstraction, making it easier to change the underlying table structure without affecting the applications that use the view.
- They can be used to restrict access to sensitive data by only granting privileges on the view.
Introduction to Materialized Views
A materialized view is a physical table that stores the result-set of a query. Unlike views, materialized views store data in a separate table, which can be indexed and optimized for faster queries. When you create a materialized view, Oracle executes the query and stores the result-set in the materialized view table.
Materialized views are useful when:
- You need to improve query performance by reducing the number of joins and subqueries.
- You want to provide a snapshot of data at a particular point in time.
- You need to support data warehousing and business intelligence applications that require fast access to large amounts of data.
Key Differences
Here are the main differences between views and materialized views:
- Data Storage: Views do not store data, while materialized views store data in a separate table.
- Query Execution: Views execute the query every time they are accessed, while materialized views store the result-set and only update it periodically or on demand.
- Performance: Materialized views can provide faster query performance, especially for complex queries or large datasets.
- Data Freshness: Views always return the latest data, while materialized views may return stale data if not updated frequently enough.
Creating and Managing Materialized Views
To create a materialized view, you need to specify the query definition, the storage parameters, and the refresh options. You can use the CREATE MATERIALIZED VIEW statement to create a new materialized view. For example:
CREATE MATERIALIZED VIEW sales_summary
BUILD IMMEDIATE
REFRESH COMPLETE ON DEMAND
AS
SELECT region, product, SUM(sales) AS total_sales
FROM sales_data
GROUP BY region, product;
In this example, the sales_summary materialized view is created with a query that sums up sales data by region and product. The BUILD IMMEDIATE clause specifies that the materialized view should be populated immediately, and the REFRESH COMPLETE ON DEMAND clause specifies that the materialized view should be refreshed on demand.
Best Practices
Here are some best practices to keep in mind when using views and materialized views:
- Use views to simplify complex queries and provide a layer of abstraction.
- Use materialized views to improve query performance and support data warehousing applications.
- Regularly update materialized views to ensure data freshness.
- Monitor query performance and adjust the materialized view refresh frequency accordingly.
In conclusion, views and materialized views are powerful tools in Oracle that can help simplify complex queries, improve data access, and support business intelligence applications. By understanding the differences between them and using them effectively, you can optimize your database design and improve overall system performance.