presto-main/src/main/java/io/prestosql/operator/TaskContext.java/TaskContext/getOutputPositions
class TaskContext:    public CounterStat getOutputPositions()
    {
        CounterStat stat = new CounterStat();
        for (PipelineContext pipelineContext : pipelineContexts) {
            if (pipelineContext.isOutputPipeline()) {
                stat.merge(pipelineContext.getOutputPositions());
            }
        }
        return stat;
    }