Hi,

Currently, when you try to add a nullable column inside a mergetree sorting index, you have an error:
Sorting key cannot contain nullable columns

Nevertheless, if you modify the code for allowing them, everything sems to be working.
So, is there still a technical reason to do allow them?

The only issue I found seems to be the case of Nullable String with collation, but I am not sure that they could be used in a sorting index case.