Formula Caching and Query Scope

The Analysis Services formula engine's ability to cache the results of calculated members can be crucial to the overall performance of your cube, but it's all too easy to turn off this caching. I've blogged already about the impact that subselects have on this and today I thought I'd discuss how you need to be careful using different calculation contexts. This is a topic that's covered in the Analysis Services 2008 Performance Guide, but I thought it would be worth talking about here because it does deserve a lot more visibility as an important cause of poor query performance, especially in SSRS reports with hand-coded MDX.

As you know, you can define calculated members in three different places in Analysis Services: on the cube (the global context), within the session (the session context) and in the WITH clause of a query (the query context). The important paragraph from the Performance Guide is this one: