code: func (p *Sort) PruneColumns(parentUsedCols []*expression.Column) {
  	child := p.GetChildByIndex(0).(LogicalPlan)
  	for _, item := range p.ByItems {
  		parentUsedCols = append(parentUsedCols, expression.ExtractColumns(item.Expr)...)
  	}
  	child.PruneColumns(parentUsedCols)
  	p.SetSchema(p.GetChildByIndex(0).GetSchema())
  }
docs: // PruneColumns implements LogicalPlan interface.