default void materialize(List<ColumnSchema> schemas) {
        for (Attr attr : attr()) {
            attr.materialize(schemas);
        }
        for (RCOperator op : children()) {
            op.materialize(schemas);
        }
    }
Make all attrs in this op and its children point to the real columns.
indexr-segment/src/main/java/io/indexr/segment/rc/RCOperator.java