code: func (col *Column) HashCode() []byte {
  	if len(col.hashcode) != 0 {
  		return col.hashcode
  	}
  	col.hashcode, _ = codec.EncodeValue(col.hashcode, types.NewStringDatum(col.FromID), types.NewIntDatum(int64(col.Position)))
  	return col.hashcode
  }
docs: // HashCode implements Expression interface.