Error occurs when using union in query

[Ted-Jiang]

Describe the bug
in pg

postgres=# select 1 union select 2;
 ?column?
----------
        1
        2
(2 rows)

in df

❯ select 1;
+----------+
| Int64(1) |
+----------+
| 1        |
+----------+
1 row in set. Query took 0.002 seconds.
❯ select 2;
+----------+
| Int64(2) |
+----------+
| 2        |
+----------+
1 row in set. Query took 0.001 seconds.
❯ select 1 union select 2;
Plan("UNION ALL schemas are expected to be the same")

I think there two problems:

1. can not get result.
2. use union but get UNION ALL: i think they are totaly different

related to #2032
To Reproduce
Steps to reproduce the behavior:

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.

[Ted-Jiang]

I will try to fix this😊

[xudong963]

@Ted-Jiang I think there is a duplicate issue: #1818

[Ted-Jiang]

@Ted-Jiang I think there is a duplicate issue: #1818

Thanks for your info❤️

[gandronchik]

It doesn't work in the current version.
select 1 union select 2;
ArrowError(ExternalError(Execution("Arrow error: External error: Internal error: Unsupported data type in hasher: Null. This was likely caused by a bug in DataFusion's code and we would welcome that you file an bug report in our issue tracker")))

SELECT 1 a UNION ALL SELECT 2;

+-------------+
| placeholder |
+-------------+
|             |
| 1           |
+-------------+

I fixed it. Now result is:
select 1 union select 2;

+----------+
| Int64(1) |
+----------+
| 2        |
| 1        |
+----------+

SELECT 1 a UNION ALL SELECT 2;

+---+
| a |
+---+
| 1 |
| 2 |
+---+