presto-main/src/main/java/io/prestosql/operator/PipelineContext.java/PipelineContext/getOutputDataSize
class PipelineContext:    public CounterStat getOutputDataSize()
    {
        CounterStat stat = new CounterStat();
        stat.merge(outputDataSize);
        for (DriverContext driver : drivers) {
            stat.merge(driver.getOutputDataSize());
        }
        return stat;
    }