Views in SQL Server are a powerful tool that allow you to simplify complex queries and provide a way to encapsulate logic. However, there are certain limitations when it comes to creating indexed views, and one of them is the inability to use the UNION operation.
Let’s take a look at an example to understand why UNION is not allowed in indexed views. Consider a scenario where we have a table called “mySampleTable” with columns ID1, ID2, and SomeData. We want to create a view called “SampleView” that includes rows from “mySampleTable” where ID1 is less than 1000, as well as rows where ID2 is less than 1000. To achieve this, we use the UNION operation in the view definition.
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
However, when we try to create an index on this view, we encounter an error message stating that the UNION operation is not allowed in indexed views. The reason behind this limitation is that the UNION operation combines the result sets of two or more SELECT statements into a single result set. This makes it difficult for SQL Server to optimize the query execution and maintain the integrity of the indexed view.
Instead of using UNION, we can rewrite the view definition using the OR operator to achieve the same result:
ALTER VIEW SampleView WITH SCHEMABINDING AS
SELECT ID1, ID2, SomeData
FROM dbo.mySampleTable
WHERE ID1 < 1000 OR ID2 < 1000
By using the OR operator, we eliminate the need for the UNION operation and can now create an index on the view successfully.
It’s important to note that while we were able to rewrite the script in this example, there may be cases where it is not possible to do so. In such cases, you may have to consider alternative approaches or avoid using indexed views altogether.
The limitations of UNION in indexed views are in place to ensure the performance and integrity of the database. By understanding these limitations, you can make informed decisions when designing your database schema and choosing the appropriate tools for your needs.
What do you think is the reason behind these limitations? Why is UNION not allowed in indexed views? I would love to hear your thoughts and insights on this topic. Please share your answer in the comments below, and I will publish your response with due credit on the blog.