How does declaring the hierarchy and level in SSAS help to improve MDX Query performance?

Question

I use SSAS in SQL Server 2008 R2 and I'm reading the book Professional Microsoft SQL Server Analysis Services 2008 with MDX Wrox Programmer to Programmer .

In chapter 5 on page 132 it says that declaring "hierarchies and level" is better for query performance. Can someone explain why this is?

Answer 1

If SSAS server knows the relationships between levels, it can do optimizations to reduce the sets it is dealing.

For example, if you have a hierarchy with sales manager and clients, by knowing the sales manager, cube can reduce the number of clients checked. This can be done on the dimension itself without going to the facts.