Function max_column_id 

pub fn max_column_id(columns: &[ColumnCatalog]) -> ColumnId

FIXME: Perhaps we should use sth like ColumnIdGenerator::new_alter, However, the SourceVersion is problematic: It doesn’t contain next_col_id. (But for now this isn’t a large problem, since drop column is not allowed for source yet..)

Besides, the logic of column id handling is a mess. In some places, we use ColumnId::placeholder(), and use col_id_gen to fill it at the end; In other places, we create column id ad-hoc.