Note that ALTER TABLE cannot alter generated stored columns

I was trying to change the generation expression of a generated stored column using ALTER TABLE. It took me a while before I figured SET GENERATED applies only to generated identity columns. I think it'd be useful to note this in the documentation with something like the attached. 0001-doc-Note

Re: Note that ALTER TABLE cannot alter generated stored columns

> I don't follow why you put this documentation under ALTER TABLE ... DROP > EXPRESSION. That action doesn't seem related to what you were hoping to do. I thought that's where it would be if we had ADD/SET EXPRESSION statements, just like the ADD/SET GENERATED statements just below.