Views in SQL Server provide a way to simplify complex queries and encapsulate logic for reuse. However, there are certain limitations to using views, one of which is the difficulty of adding columns to them. In this article, we will explore why adding columns to views can be expensive and discuss alternative solutions.

When we need to add a column to a regular query or stored procedure, we can simply modify the query and include the new column. However, with views, we need to modify the view itself. This means that any query using the view will now have the additional column’s data, even if it doesn’t need it. This can lead to increased network traffic and reduced performance in the parts where the view is used.

One might think that the solution is to add the column outside the view. However, this can be very expensive. Let’s consider an example using the AdventureWorks sample database. We have a view called “vw_ViewLimit1” that selects data from multiple tables. If we want to retrieve an additional column from one of these tables, we can either modify the view or use a regular T-SQL query.

Comparing the performance of the view and the regular T-SQL query, we find that they have the same execution plan and query cost. However, if we try to retrieve the additional column that is not in the view, we notice that the view becomes more expensive compared to the regular T-SQL query. This is because the view needs to perform a join to retrieve the additional column, whereas the regular query can simply add the column to the statement.

So, why is adding columns to views more expensive? The reason is that views are precompiled and optimized, and any changes to the view require recompilation. This can be a costly operation, especially if the view is used at multiple places in the code. On the other hand, regular T-SQL queries are compiled and optimized at runtime, allowing for more flexibility in adding columns.

It’s important to note that if the view is not used at multiple places, modifying it to add a column may not cause any performance loss. However, in cases where the view is widely used, it’s advisable to consider alternative solutions. One such solution is to use joins in the regular T-SQL queries to add the additional column when needed.

In conclusion, adding columns to views in SQL Server can be expensive due to the need for recompilation and the potential impact on performance. It’s important to carefully consider the trade-offs and alternatives when working with views in order to maintain optimal performance in your database applications.