@Test
  public final void testNext() throws IOException, PlanningException {
    FileFragment[] frags =
        StorageManager.splitNG(
            conf, "employee", employee.getMeta(), employee.getPath(), Integer.MAX_VALUE);
    Path workDir = new Path(testDir, TestExternalSortExec.class.getName());
    TaskAttemptContext ctx =
        new TaskAttemptContext(
            conf,
            LocalTajoTestingUtility.newQueryUnitAttemptId(),
            new FileFragment[] {frags[0]},
            workDir);
    ctx.setEnforcer(new Enforcer());
    Expr expr = analyzer.parse(QUERIES[0]);
    LogicalPlan plan = planner.createPlan(expr);
    LogicalNode rootNode = plan.getRootBlock().getRoot();

    PhysicalPlanner phyPlanner = new PhysicalPlannerImpl(conf, sm);
    PhysicalExec exec = phyPlanner.createPlan(ctx, rootNode);

    ProjectionExec proj = (ProjectionExec) exec;

    // TODO - should be planed with user's optimization hint
    if (!(proj.getChild() instanceof ExternalSortExec)) {
      UnaryPhysicalExec sortExec = proj.getChild();
      SeqScanExec scan = sortExec.getChild();

      ExternalSortExec extSort =
          new ExternalSortExec(ctx, sm, ((MemSortExec) sortExec).getPlan(), scan);
      proj.setChild(extSort);
    }

    Tuple tuple;
    Tuple preVal = null;
    Tuple curVal;
    int cnt = 0;
    exec.init();
    long start = System.currentTimeMillis();
    TupleComparator comparator =
        new TupleComparator(
            proj.getSchema(),
            new SortSpec[] {
              new SortSpec(new Column("managerId", Type.INT4)),
              new SortSpec(new Column("empId", Type.INT4))
            });

    while ((tuple = exec.next()) != null) {
      curVal = tuple;
      if (preVal != null) {
        assertTrue(
            "prev: " + preVal + ", but cur: " + curVal, comparator.compare(preVal, curVal) <= 0);
      }
      preVal = curVal;
      cnt++;
    }
    long end = System.currentTimeMillis();
    assertEquals(numTuple, cnt);

    // for rescan test
    preVal = null;
    exec.rescan();
    cnt = 0;
    while ((tuple = exec.next()) != null) {
      curVal = tuple;
      if (preVal != null) {
        assertTrue(
            "prev: " + preVal + ", but cur: " + curVal, comparator.compare(preVal, curVal) <= 0);
      }
      preVal = curVal;
      cnt++;
    }
    assertEquals(numTuple, cnt);
    exec.close();
    System.out.println("Sort Time: " + (end - start) + " msc");
  }