I was reviewing the SQL Server physical operators listed on TechNet (don't judge, you know you've done it) and [read that the Hash Match physical operator is sometimes used to implement the `UNION` logical operator](https://technet.microsoft.com/en-us/library/ms189582(v=sql.105).aspx).

I have never seen that done and would like to learn more. An example query would be great. When is it used and when is it better than the alternatives? (Those are usually the same, but not always.)