Erik Darling has a tale to tell:
The only columns that we were really selecting from the Comments table were UserId and CreationDate, which are an integer and a datetime.
Those are relatively easy columns to deal with, both from the perspective of reading and sorting.
In order to show you how column selection can muck things up, we need to create a more appropriate column store index, add columns to the select list, and use a where clause to restrict the number of rows we’re sorting. Otherwise, we’ll get a 16GB memory grant for every query.
Read on to see how one little (or, well, big) string column can foul up the whole works.