Get most out of partition slices

Setting partition slice has always been an important optimization technique in Analysis Services. Every presentation talked about it and every whitepaper mentioned it, for example the Microsoft SQL Server 2000 Analysis Services Performance Guide contains a chapter appropriately named “Define the Data Slice for Each Partition”, here is a quote from it:

“If the data slice value for a partition is set properly, Analysis Services can quickly eliminate irrelevant partitions from the query processing and significantly reduce the amount of physical I/O and processor time needed for many queries issued against MOLAP and HOLAP partitions <skip>

Caution: Creating a partition without setting the data slice is not a good practice, and can result in considerable overhead being added to the system (artificially increasing response times). Without the data slice, Analysis Services cannot limit a query to the appropriate partitions and must scan each partition even if zero cells will be returned.

The data slice enables Analysis Services to determine which partitions contain data relevant to the query”

However, after AS2005 was released, there was a new tip making it into presentations - “For MOLAP partitions no need to specify slice property, it is detected automatically”. Even though I briefly debunked it before here,  this rumor just kept popping up, and it even made it as far as official “OLAP Design Best Practices for Analysis Services 2005” document – here is the quote: