code: func (p *PhysicalAggregation) MarshalJSON() ([]byte, error) {
  	buffer := bytes.NewBufferString("{")
  	aggFuncs, err := json.Marshal(p.AggFuncs)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	gbyExprs, err := json.Marshal(p.GroupByItems)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	buffer.WriteString(fmt.Sprintf(
  		"\"AggFuncs\": %s,\n"+
  			"\"GroupByItems\": %s,\n"+
  			"\"child\": \"%s\"}", aggFuncs, gbyExprs, p.children[0].GetID()))
  	return buffer.Bytes(), nil
  }
docs: // MarshalJSON implements json.Marshaler interface.