function testExec_SingleColumn() {
  var inputRelation = lf.proc.Relation.fromRows(
      dataGenerator.sampleEmployees, [e.getName()]);
  var childStep = new lf.proc.NoOpStep([inputRelation]);
  var groupByStep = new lf.proc.GroupByStep([e.jobId]);
  groupByStep.addChild(childStep);

  var employeesPerJob = dataGenerator.employeeGroundTruth.employeesPerJob;
  return groupByStep.exec().then(function(relations) {
    var jobIds = employeesPerJob.getKeys();
    assertEquals(jobIds.length, relations.length);
    relations.forEach(function(relation) {
      var jobId = /** @type {string} */ (relation.entries[0].getField(e.jobId));
      var expectedRows = employeesPerJob.get(jobId);
      assertEquals(expectedRows.length, relation.entries.length);
      relation.entries.forEach(function(entry) {
        assertEquals(jobId, entry.getField(e.jobId));
      });
    });
  });
}
Tests GroupByStep#exec() method for the case where grouping is performed on a
single column.
@return {!IThenable}