code: func columnNamesCover(normalColNames map[string]struct{}, dependColNames map[string]struct{}) error {
  	for name := range dependColNames {
  		if _, ok := normalColNames[name]; !ok {
  			return errBadField.GenWithStackByArgs(name, "generated column function")
  		}
  	}
  	return nil
  }
docs: // columnNamesCover checks whether dependColNames is covered by normalColNames or not.
  // it's only for alter table add column because before alter, we can make sure that all
  // columns in table are verified already.