fix: union schema

[gandronchik]

fix UNION schema

example:
SELECT 1 a UNION ALL SELECT 2;

Closes #2083

[jackwener]

It's better to link the related issue if it exists.

If it doesn't exist, I think it would be better to create it and link it.

[alamb]

Thank you @gandronchik I tried this out locally and it seems to have worked well 👍

Nice work

❯ SELECT 1 a UNION ALL SELECT 2;
+---------+
| column0 |
+---------+
| 2       |
| 1       |
+---------+
2 rows in set. Query took 0.048 seconds.

I also verified that this behavior (taking the left schema as the output schema) is consistent with postgres:

alamb=# SELECT 1 as a UNION ALL SELECT 2;
 a 
---
 1
 2
(2 rows)

alamb=# SELECT 1 as a UNION ALL SELECT 2 as b;
 a 
---
 1
 2
(2 rows)

The only thing I would like to see is a specific test. The other comments are just suggestions for improvements but not required in my opinion.

[andygrove]

Hi @gandronchik Do you have time to address the PR feedback here so that we can get your change approved + merged?

[alamb]

Thanks @gandronchik -- looks good now