code: func getConstantColumnID(e []expression.Expression) int64 {
  	if len(e) != 2 {
  		return unknownColumnID
  	}
  	col, ok1 := e[0].(*expression.Column)
  	_, ok2 := e[1].(*expression.Constant)
  	if ok1 && ok2 {
  		return col.ID
  	}
  	col, ok1 = e[1].(*expression.Column)
  	_, ok2 = e[0].(*expression.Constant)
  	if ok1 && ok2 {
  		return col.ID
  	}
  	return unknownColumnID
  }
docs: // getConstantColumnID receives two expressions and if one of them is column and another is constant, it returns the
  // ID of the column.