what is index and can non-clustered index be non-unique?
问题 Subquestion to my question [1]: All definitions of (MS SQL Server) index (that I could find) are ambiguous and all explanations, based on it, narrate something using undefined or ambiguously defined terms. What is the definition of index? For ex., the most common definition of index from wiki (http://en.wikipedia.org/wiki/Index_(database) ) : 1) "A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and