presto-main/src/main/java/io/prestosql/operator/DriverContext.java/DriverContext/getInputPositions
class DriverContext:    public CounterStat getInputPositions()
    {
        OperatorContext inputOperator = getFirst(operatorContexts, null);
        if (inputOperator != null) {
            return inputOperator.getInputPositions();
        }
        else {
            return new CounterStat();
        }
    }