Function validate_column_default 

pub fn validate_column_default(
    context: &ColumnAlterAnalysisContext<'_>,
    table: &str,
    column: &str,
    default: &mut Expr,
) -> Result<bool, SQLError>

ATExecColumnDefault for SET DEFAULT: the column is checked before the default is analyzed against its type. Returns whether a default remains; SET DEFAULT NULL leaves none.