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Indexed views in SQL server

Indexed views in SQL Server are database objects that store the results of a query as a physical structure in the database. They are created by persisting the computed result of a view into a clustered index and, optionally, nonclustered indexes. This allows for improved query performance by avoiding the need to recompute the result every time the view is accessed.

Here are some key points about indexed views in SQL Server:

  1. Materialized Views: Indexed views are sometimes referred to as materialized views because they store the actual data in a physical structure rather than just defining a query.
  2. Query Performance: When a query references an indexed view, SQL Server can use the precomputed results stored in the view to satisfy the query, eliminating the need to execute the underlying query and perform complex calculations or joins. This can significantly improve performance for queries that involve aggregations, joins, or complex calculations.
  3. Indexing: Indexed views have a clustered index, which determines the physical order of the rows in the view. Additionally, nonclustered indexes can be created on the indexed view to further enhance query performance.
  4. Query Restrictions: There are certain restrictions on the types of queries that can benefit from indexed views. The query must reference the indexed view directly and meet specific criteria, such as using only deterministic functions, not including outer joins or self-joins, and adhering to certain constraints.
  5. View Maintenance: SQL Server automatically maintains the indexed view when underlying data changes. This means that the view needs to be updated whenever the underlying tables are modified. The system handles these updates to keep the view consistent with the underlying data.
  6. Considerations and Limitations: Indexed views can provide significant performance benefits, but they also introduce overhead during data modifications. The indexes on the view need to be updated, which can impact the overall system performance. Additionally, indexed views require disk space to store the materialized data, so space considerations should be taken into account.

Indexed views are most effective in scenarios where the underlying data changes infrequently, but queries against the view are frequent and performance-sensitive. They can be a powerful tool to improve query performance and should be used judiciously based on the specific requirements and characteristics of your application.

Let's walk through an example to understand how indexed views work in SQL Server.

Suppose we have a database with two tables: "Orders" and "Customers." The "Orders" table contains information about customer orders, including the order ID, customer ID, order date, and order amount. The "Customers" table contains customer information, including the customer ID, name, and address.

We want to create an indexed view that provides a summary of total order amounts for each customer. Here's how we can accomplish that:

1-Create the View: First, we create a regular view that calculates the total order amount for each customer. The view definition would look like this:


CREATE VIEW vw_OrderTotals
AS
SELECT Customers.CustomerID, Customers.Name, SUM(Orders.OrderAmount) AS TotalAmount
FROM Customers
JOIN Orders ON Customers.CustomerID = Orders.CustomerID
GROUP BY Customers.CustomerID, Customers.Name;

This view retrieves data from both the "Customers" and "Orders" tables, performs a join, and calculates the sum of the order amounts for each customer.

2-Create the Indexed View: To convert the regular view into an indexed view, we need to create a clustered index on the view. Here's how we can do that:


CREATE UNIQUE CLUSTERED INDEX idx_OrderTotals ON vw_OrderTotals (CustomerID);

This statement creates a clustered index on the "CustomerID" column of the view. The clustered index determines the physical order of the rows in the indexed view.

3-Querying the Indexed View: Once the indexed view is created, we can query it just like any other table. For example, to retrieve the total order amount for a specific customer, we can execute the following query:


SELECT TotalAmount
FROM vw_OrderTotals
WHERE CustomerID = 123;

This query will retrieve the precomputed total order amount for the customer with the ID 123 directly from the indexed view, without needing to execute the original join and aggregation.

4-Maintaining the Indexed View: SQL Server automatically maintains the indexed view when the underlying data changes. If a new order is inserted or an existing order is updated or deleted, the indexed view will be updated to reflect the changes.

It's important to note that indexed views are beneficial when the view is frequently queried, and the underlying data changes infrequently. This way, the cost of maintaining the indexed view is minimized, and the performance gains from precomputed results are maximized.

Indexed views can provide significant performance improvements for complex queries involving aggregations, joins, and calculations, as the results are already materialized and indexed. However, it's essential to carefully consider the trade-offs and ensure that the view and query meet the necessary criteria for indexing.

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