@Override
  public Expression resolveSelect(final int idx, final Expression expression) {
    final Expression resolved = columnMappings.get(idx);
    return resolved == null ? expression : resolved;
  }
@Test
  public void shouldResolveNoneUdtfSelectExpressionToSelf() {
    // Given:
    final Expression exp = mock(Expression.class);

    // When:
    final Expression result = flatMapNode.resolveSelect(0, exp);

    // Then:
    assertThat(result, is(exp));
  }