code: func (col *Column) EvalReal(ctx sessionctx.Context, row chunk.Row) (float64, bool, error) {
  	if row.IsNull(col.Index) {
  		return 0, true, nil
  	}
  	if col.GetType().Tp == mysql.TypeFloat {
  		return float64(row.GetFloat32(col.Index)), false, nil
  	}
  	return row.GetFloat64(col.Index), false, nil
  }
docs: // EvalReal returns real representation of Column.