presto-main/src/main/java/io/prestosql/sql/planner/PartitioningHandle.java/PartitioningHandle/toString
class PartitioningHandle:    @Override
    public String toString()
    {
        if (connectorId.isPresent()) {
            return connectorId.get() + ":" + connectorHandle;
        }
        return connectorHandle.toString();
    }