Indexed views are special views that can offer huge performance improvements over regular views. They perform faster because they are stored in the database the same way a table is stored. They also have a unique clustered index which greatly improves performance. However, these benefits come at a price. Indexed views have a lot of limitations to consider before implementation.
To see the full list of limitations and to learn more about indexed views, click here:
Recently, I was creating several indexed views and came across a limitation of joining to the same dimension table multiple times (role-playing dimension). I received the following error message: Cannot create index on view “x_DW.dbo.vw_SchemaBoundView_Test”. The view contains a self join on “x_DW.dbo.DimUser”.
Google searching resulted in several not-so-great suggestions to overcome this limitation. I believe the best solution was to create a new table and essentially writing duplicate data values to this new table. For example; create a dbo.DimUser2 table. I really do not like this approach because I would have to create another table for one single purpose and maintain the ETL and data for this one purpose.
After some thinking, I came up with another solution which requires less maintenance and seems to have good performance with less overhead. I broke my views up into two views: ViewName and ViewName_Base. The Base view has essentially everything minus the second join to the same table. I identified which of the two self-joining tables had the greatest amount of columns or caused the biggest performance hit. This was the join I included in my Base view because it will get stored like a table and indexed. I created the base view with schemabinding and created the unique clustered index.
Next, I created the other, non-base, view. This was nearly identical to the Base view. However, it’s primary source is the Base view. I then joined the Base view to the other self-joining table reference to get the final desired columns which I needed in my select statement.
By splitting the original view into two views, I was able to work around the self-joining table limitation in indexed views but was still able to have a really nice performance improvement. I did not need to create additional ETL or need to create another table with duplicate data that I would need to maintain.