How to quickly select DISTINCT dates from a Date/Time field, SQL Server

Question

I am wondering if there is a good-performing query to select distinct dates (ignoring times) from a table with a datetime field in SQL Server.

My problem isn\'t getting

Answer 1

What is your predicate on that other filtered column ? Have you tried whether you get improvement from an index on that other filtered column, followed by the datetime field ?

I'm largely guessing here, but 5 seconds to filter a set of perhaps 100000 rows down to 40000 and then doing a sort (which is presumably what goes on) doesn't seem like an unreasonable time to me. Why do you say it's too slow ? Because it doesn't match expectations ?