Kirill Bondar 1/11/2012 6:41:58 AM
The functionality is split between two products: the function is coded using .NET and they have documented an attribute to specify whether aggregate function is invariant to order of the data or not, but documentation states it is "reserved for future use". Yet, the attribute by itself does nothing - it is merely a flag for SQL to know whether the data it feeds to a function needs an ordering or not - but SQL documentation says nothing about the attribute.
Upcoming SQL 2012 release notes mention some improvements they've made for aggregate functions but its not clear whether they are applicable for custom aggregates. Yet the product is in early stage and documentation is obviously is not complete.