code: func (p *Trim) PruneColumns(parentUsedCols []*expression.Column) {
  	used := getUsedList(parentUsedCols, p.schema)
  	for i := len(used) - 1; i >= 0; i-- {
  		if !used[i] {
  			p.schema.Columns = append(p.schema.Columns[:i], p.schema.Columns[i+1:]...)
  		}
  	}
  	p.GetChildByIndex(0).(LogicalPlan).PruneColumns(parentUsedCols)
  }
docs: // PruneColumns implements LogicalPlan interface.