code: func (col *CorrelatedColumn) EvalDecimal(ctx sessionctx.Context, row chunk.Row) (*types.MyDecimal, bool, error) {
  	if col.Data.IsNull() {
  		return nil, true, nil
  	}
  	return col.Data.GetMysqlDecimal(), false, nil
  }
docs: // EvalDecimal returns decimal representation of CorrelatedColumn.