Alias column name is not recognized with union operator

[ramanich1]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select 1 as a,(select a union select a);

2. What did you expect to see? (Required)

+---+---------------------------+
| a | (select a union select a) |
+---+---------------------------+
| 1 |                         1 |
+---+---------------------------+
1 row in set (0.04 sec)

3. What did you see instead (Required)

ERROR 1054 (42S22): Unknown column 'a' in 'field list'

4. What is your TiDB version? (Required)

+-------------------------+--------------------------------------------------------------------------+
| Variable_name           | Value                                                                    |
+-------------------------+--------------------------------------------------------------------------+
| innodb_version          | 5.6.25                                                                   |
| protocol_version        | 10                                                                       |
| tidb_analyze_version    | 2                                                                        |
| tidb_row_format_version | 2                                                                        |
| tls_version             | TLSv1,TLSv1.1,TLSv1.2                                                    |
| version                 | 5.7.25-TiDB-v5.2.1                                                       |
| version_comment         | TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible |
| version_compile_machine | x86_64                                                                   |
| version_compile_os      | osx10.8                                                                  |
+-------------------------+--------------------------------------------------------------------------+

[yudongusa]

Lower severity to moderate as this is not blocking issue, and will be resolved with moderate optimizer priority.

[winoros]

It's a duplicate issue with #8190. We need an elegant code refactoring to solve this problem.
You can track the #8190 instead.
So closed since it's duplicated.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.