Evgeny (6/3/2013)
Did you do any tests about?
You mean other than the ones on the blog post I referenced?
Particullary for SQL Server 2012...
No, but neither the index architecture nor the methods for aggregating have changed in 2012, so will be the same as 2008. As I said, you want key ideally to provide the sort for a stream aggregate, but it depends entirely on what else is in the index and what the query looks like.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability