Update: Please read the summary post of all the 11 Limitations of the view SQL SERVER – The Limitations of the Views – Eleven and more…
If you want to create an Indexed View, you ought to know that UNION Operation is not allowed in Indexed View. It is quite surprising at times when the UNION operation looks very innocent and seems that it cannot be used in the View.
Before an in-depth understanding this subject, let me show you a script where UNION is not allowed in Indexed View:
USE tempdb
GO
IF EXISTS (SELECT * FROM sys.views WHERE OBJECT_ID = OBJECT_ID(N'[dbo].[SampleView]'))
DROP VIEW [dbo].[SampleView]
GO
IF EXISTS (SELECT * FROM sys.objects WHERE OBJECT_ID = OBJECT_ID(N'[dbo].[mySampleTable]') AND TYPE IN (N'U'))
DROP TABLE [dbo].[mySampleTable]
GO
-- Create SampleTable
CREATE TABLE mySampleTable (ID1 INT, ID2 INT, SomeData VARCHAR(100))
INSERT INTO mySampleTable (ID1,ID2,SomeData)
SELECT TOP 100000 ROW_NUMBER() OVER (ORDER BY o1.name),
ROW_NUMBER() OVER (ORDER BY o2.name),
o2.name
FROM sys.all_objects o1
CROSS JOIN sys.all_objects o2
GO
-- Create View
CREATE VIEW SampleView
WITH SCHEMABINDING
AS
SELECT ID1,ID2,SomeData
FROM dbo.mySampleTable
WHERE ID1 < 1000
UNION
SELECT ID1,ID2,SomeData
FROM dbo.mySampleTable
WHERE ID2 < 1000
GO
-- Create Index on View
CREATE UNIQUE CLUSTERED INDEX [IX_ViewSample] ON [dbo].[SampleView]
(
ID2 ASC
)
GO
/* Above statement will thrown an error
Msg 10116, Level 16, State 1, Line 1
Cannot create index on view 'tempdb.dbo.SampleView' because it contains one or more UNION, INTERSECT, or EXCEPT operators. Consider creating a separate indexed view for each query that is an input to the UNION, INTERSECT, or EXCEPT operators of the original view.
*/
-- Aleter View to replace COUNT with BIG_COUNT
ALTER VIEW SampleView
WITH SCHEMABINDING
AS
SELECT ID1,ID2,SomeData
FROM dbo.mySampleTable
WHERE ID1 < 1000 OR ID2 < 1000
GO
-- Now let us create Index again - this time successfully
CREATE UNIQUE CLUSTERED INDEX [IX_ViewSample] ON [dbo].[SampleView]
(
ID2 ASC
)
GO
During the script, the following ERROR would occur if you try to create the index while UNION operation is in the view:
Msg 10116, Level 16, State 1, Line 1
Cannot create index on view ‘tempdb.dbo.SampleView’ because it contains one or more UNION, INTERSECT, or EXCEPT operators. Consider creating a separate indexed view for each query that is an input to the UNION, INTERSECT, or EXCEPT operators of the original view.
In contrast to this converting the UNION to OR operation would give the same result, plus it would allow you to create an index on the View. Well, our example is one in which we are able to re-write the script with OR clause. However, keep in mind that there can be cases where it is not possible to re-write and you might end up not using Views with Index.
In this series, I have been writing about many limitations and their explanation. Now here are my questions for you:
- What do you think is the reason behind these limitations?
- Why UNION is not allowed in the View?
I will publish your answer with due credit on the blog.
Reference : Pinal Dave (https://blog.sqlauthority.com)
