presto-main/src/main/java/io/prestosql/sql/planner/PlanFragment.java/PlanFragment/toString
class PlanFragment:    @Override
    public String toString()
    {
        return toStringHelper(this)
                .add("id", id)
                .add("partitioning", partitioning)
                .add("partitionedSource", partitionedSources)
                .add("partitionFunction", partitioningScheme)
                .toString();
    }