code: func (s *exprSet) Slice() []Expression {
  	if s.constfalse {
  		return []Expression{&Constant{
  			Value:   types.NewDatum(false),
  			RetType: types.NewFieldType(mysql.TypeTiny),
  		}}
  	}
  
  	idx := 0
  	for i := 0; i < len(s.data); i++ {
  		if !s.tombstone[i] {
  			s.data[idx] = s.data[i]
  			idx++
  		}
  	}
  	return s.data[:idx]
  }
docs: // Slice returns the valid expressions in the exprSet, this function has side effect.