func loadPredicateData(pred *common.Predicate) (*predicateData, error) {
	p := predicateData{Predicate: pred}

	// Parse the input value given to make sure it matches up with the field
	// type.
	switch pred.Field {
	case common.SPAN_ID:
		// Span IDs are sent as hex strings.
		var id common.SpanId
		if err := id.FromString(pred.Val); err != nil {
			return nil, errors.New(fmt.Sprintf("Unable to parse span id '%s': %s",
				pred.Val, err.Error()))
		}
		p.key = id.Val()
		break
	case common.DESCRIPTION:
		// Any string is valid for a description.
		p.key = []byte(pred.Val)
		break
	case common.BEGIN_TIME, common.END_TIME, common.DURATION:
		// Parse a base-10 signed numeric field.
		v, err := strconv.ParseInt(pred.Val, 10, 64)
		if err != nil {
			return nil, errors.New(fmt.Sprintf("Unable to parse %s '%s': %s",
				pred.Field, pred.Val, err.Error()))
		}
		p.key = u64toSlice(s2u64(v))
		break
	case common.TRACER_ID:
		// Any string is valid for a tracer ID.
		p.key = []byte(pred.Val)
		break
	default:
		return nil, errors.New(fmt.Sprintf("Unknown field %s", pred.Field))
	}

	// Validate the predicate operation.
	switch pred.Op {
	case common.EQUALS, common.LESS_THAN_OR_EQUALS,
		common.GREATER_THAN_OR_EQUALS, common.GREATER_THAN:
		break
	case common.CONTAINS:
		if p.fieldIsNumeric() {
			return nil, errors.New(fmt.Sprintf("Can't use CONTAINS on a "+
				"numeric field like '%s'", pred.Field))
		}
	default:
		return nil, errors.New(fmt.Sprintf("Unknown predicate operation '%s'",
			pred.Op))
	}

	return &p, nil
}
func (shd *shard) FindSpan(sid common.SpanId) *common.Span {
	lg := shd.store.lg
	primaryKey := append([]byte{SPAN_ID_INDEX_PREFIX}, sid.Val()...)
	buf, err := shd.ldb.Get(shd.store.readOpts, primaryKey)
	if err != nil {
		if strings.Index(err.Error(), "NotFound:") != -1 {
			return nil
		}
		lg.Warnf("Shard(%s): FindSpan(%s) error: %s\n",
			shd.path, sid.String(), err.Error())
		return nil
	}
	var span *common.Span
	span, err = shd.decodeSpan(sid, buf)
	if err != nil {
		lg.Errorf("Shard(%s): FindSpan(%s) decode error: %s\n",
			shd.path, sid.String(), err.Error())
		return nil
	}
	return span
}
func (shd *shard) FindChildren(sid common.SpanId, childIds []common.SpanId,
	lim int32) ([]common.SpanId, int32, error) {
	searchKey := append([]byte{PARENT_ID_INDEX_PREFIX}, sid.Val()...)
	iter := shd.ldb.NewIterator(shd.store.readOpts)
	defer iter.Close()
	iter.Seek(searchKey)
	for {
		if !iter.Valid() {
			break
		}
		if lim == 0 {
			break
		}
		key := iter.Key()
		if !bytes.HasPrefix(key, searchKey) {
			break
		}
		id := common.SpanId(key[17:])
		childIds = append(childIds, id)
		lim--
		iter.Next()
	}
	return childIds, lim, nil
}