@Test
public void testCountUnique() {

  column1.append(LocalDateTime.of(2000, 1, 1, 0, 0));
  column1.append(LocalDateTime.of(2000, 1, 1, 0, 0));
  column1.append(LocalDateTime.of(2000, 2, 1, 0, 0));
  column1.appendMissing();
  assertEquals(3, column1.countUnique());
}