    def test_time_window_coverage(self):
        """
        The size of timestamp col is less than a2 col.
        """
        p_start = Pipeline.input('a')
        p1 = p_start.flat_map('a', 'a1', lambda x: list(range(1, x + 1)))
        p2 = p_start.flat_map('a', 'a2', lambda x: list(range(1, x + 4)))
        p = (
            p1.concat(p2)
            .map('a1', 'ts', lambda x: x * 1000)
            .time_window('a1', 'b', 'ts', 3, 3, sum)
            .output('b', 'a2')
        )
        res = p(4).to_list()
        self.assertEqual(res, [[3, 1], [7, 2], [Empty(), 3], [Empty(), 4], [Empty(), 5], [Empty(), 6], [Empty(), 7]])
The size of timestamp col is less than a2 col.
tests/unittests/runtime/test_user_pipeline.py