Select result is not compatible with MySQL while performing union operation between char and integer

[PragmaTwice]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE a(b char);
SELECT b FROM a UNION SELECT avg(1);

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

In MySQL:

+---------+
| b       |
+---------+
| 1.00000 |
+---------+
1 row in set (0.00 sec)

Type of 1.00000 is char(7), not integer

3. What did you see instead (Required)

+--------+
| b      |
+--------+
| 1.0000 |
+--------+
1 row in set (0.00 sec)

Type of 1.0000 is char(24), not integer (by creating a view and describing it, strangely)

4. What is your TiDB version? (Required)

+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()
                                                                                                                                                 |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.2.0-alpha-88-ged52601e6
Edition: Community
Git Commit Hash: ed52601e6eb560138db8cdccdfa1b5e2d33a11f0
Git Branch: master
UTC Build Time: 2021-06-16 13:03:48
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[yudongusa]

seems duplicate one of #25600