presto-main/src/main/java/io/prestosql/sql/planner/StageExecutionPlan.java/StageExecutionPlan/toString
class StageExecutionPlan:    @Override
    public String toString()
    {
        return toStringHelper(this)
                .add("fragment", fragment)
                .add("splitSources", splitSources)
                .add("subStages", subStages)
                .toString();
    }