code: func (p *LogicalUnionScan) PruneColumns(parentUsedCols []*expression.Column) error {
  	for _, col := range p.Schema().TblID2Handle {
  		parentUsedCols = append(parentUsedCols, col[0])
  	}
  	return p.children[0].PruneColumns(parentUsedCols)
  }
docs: // PruneColumns implements LogicalPlan interface.