[HIVE-22601] Some columns will be lost when a UDTF has multiple aliases in some cases

[Makoto Yui]

Hi committers,

Could someone please review this ticket and my colleague's patch?
https://issues.apache.org/jira/browse/HIVE-22601

The following query produces a wrong result in the current stable
release of Hive (2.3.6 and 3.1.2).

SELECT stack(1, 'a', 'b', 'c') AS (c1, c2, c3)
UNION ALL
SELECT stack(1, 'd', 'e', 'f') AS (c1, c2, c3);

+---------+
| _u1.c3  |
+---------+
| c       |
| f       |
+---------+

The same issue happened for

select
 posexplode(split('1,4', ',')) as (seq, col)
union all
select
 posexplode(split('2,6', ',')) as (seq, col)

_u1.col
1
4
2
6

Thanks,
Makoto