code: func canProjectionBeEliminatedLoose(p *LogicalProjection) bool {
  	for _, expr := range p.Exprs {
  		_, ok := expr.(*expression.Column)
  		if !ok {
  			return false
  		}
  	}
  	return true
  }
docs: // canProjectionBeEliminatedLoose checks whether a projection can be eliminated,
  // returns true if every expression is a single column.