presto-main/src/main/java/io/prestosql/operator/AssignUniqueIdOperator.java/AssignUniqueIdOperator/getOutput
class AssignUniqueIdOperator:    @Override
    public Page getOutput()
    {
        if (inputPage == null) {
            return null;
        }

        Page outputPage = processPage();
        inputPage = null;
        return outputPage;
    }