SQL SERVER – Indexed View always Use Index on Table

This blog post is written in response to T-SQL Tuesday hosted by Shankar Reddy.

I have been recently writing about Views and their Limitations. While writing this article series, I got inspired to write about SQL Server Quiz Questions. You can view the Quiz Question posted over here.

In SQL Server 2005, a single table can have maximum 249 non clustered indexes and 1 clustered index. In SQL Server 2008, a single table can have maximum 999 non clustered indexes and 1 clustered index. It is widely believed that a table can have only 1 clustered index, and this belief is true. I have some questions for all of you. Let us assume that I am creating view from the table itself and then create a clustered index on it. In my view, I am selecting the complete table itself.

USE tempdb
GO
-- Create sample Table
CREATE TABLE mySampleTable(ID1 INT, ID2 INT, SomeData VARCHAR(100))
INSERT INTO mySampleTable(ID1,ID2,SomeData)
SELECT TOP 1000 ROW_NUMBER()OVER (ORDER BY o1.name),
ROW_NUMBER()OVER (ORDER BY o1.name DESC),
o1.name
FROM sys.all_objects o1
CROSS JOIN sys.all_objects o2
GO
-- Create Index on Table
CREATE UNIQUE CLUSTERED INDEX [IX_SampleView] ON mySampleTable
(
ID1 ASC
)
GO
-- Create sample View
CREATE VIEW SampleView
WITH SCHEMABINDING
AS
SELECT
ID1,ID2,SomeData
FROM dbo.mySampleTable
GO
-- Create Index on View
CREATE UNIQUE CLUSTERED INDEX [IX_ViewSample] ON [dbo].[SampleView]
(
ID2 ASC
)
GO
-- Enable Execution Plan using CTRL + M
SELECT ID1,ID2,SomeData
FROM mySampleTable
GO
SELECT ID1,ID2,SomeData
FROM SampleView
GO
-- Clean up
DROP VIEW SampleView
DROP TABLE mySampleTable
GO

Now run the following script and answer these questions:

Q1. Does the table use an index created on itself?
Q2. Does the view use an index created on itself?
Q3. Do both the queries use the same index? If yes, why? If no, why not?

The answers are very clear.

The answers are very clear.

A1: Yes
A2: No
A3: Read the rest of the blog! or visit BeyondRelationa.com (where you can win iPAD as well).

Usually, the assumption is that Index on table will use Index on table and Index on view will be used by view. However, that is the misconception. It does not happen this way. In fact, if you notice the image, you will find the both of them (table and view) use both the index created on the table. The index created on the view is not used. The reason for the same as listed in BOL.

The cost of using the indexed view may exceed the cost of getting the data from the base tables, or the query is so simple that a query against the base tables is fast and easy to find. This often happens when the indexed view is defined on small tables. You can use the NOEXPAND hint if you want to force the query processor to use the indexed view. This may require you to rewrite your query if you don’t initially reference the view explicitly. You can get the actual cost of the query with NOEXPAND and compare it to the actual cost of the query plan that doesn’t reference the view. If they are close, this may give you the confidence that the decision of whether or not to use the indexed view doesn’t matter.

Various answers to this question are given over here.

Reference: Pinal Dave (http://blog.SQLAuthority.com)