function testExec_MultiColumn() {
  var inputRelation = lf.proc.Relation.fromRows(
      dataGenerator.sampleJobs, [j.getName()]);
  var childStep = new lf.proc.NoOpStep([inputRelation]);
  var groupByStep = new lf.proc.GroupByStep([j.minSalary, j.maxSalary]);
  groupByStep.addChild(childStep);

  return groupByStep.exec().then(function(relations) {
    var jobCount = 0;
    relations.forEach(function(relation) {
      var groupByMinSalary = relation.entries[0].getField(j.minSalary);
      var groupByMaxSalary = relation.entries[0].getField(j.maxSalary);
      relation.entries.forEach(function(entry) {
        assertEquals(groupByMinSalary, entry.getField(j.minSalary));
        assertEquals(groupByMaxSalary, entry.getField(j.maxSalary));
        jobCount++;
      });
    });
    assertEquals(inputRelation.entries.length, jobCount);
  });
}
Tests GroupByStep#exec() method for the case where grouping is performed on
multiple columns.
@return {!IThenable}