cast length error in union clause

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

 select '123' union select cast(45678 as char);
 select '123' union select cast(45678 as char(5));
 select '123' union select cast(45678 as char(2));

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

mysql> select '123' union select cast(45678 as char);
Field   1:  `123`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_general_ci (45)
Length:     24
Max_length: 5
Decimals:   0
Flags:


+-------+
| 123   |
+-------+
| 123   |
| 45678 |
+-------+
2 rows in set (0.07 sec)

3. What did you see instead (Required)

mysql>  select '123' union select cast(45678 as char);
Field   1:  `123`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_bin (46)
Length:     12
Max_length: 3
Decimals:   31
Flags:


+------+
| 123  |
+------+
| 456  |
| 123  |
+------+
2 rows in set, 1 warning (0.00 sec)

MySQL > show warnings;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1406 | Data Too Long, field len 3, data len 5 |
+---------+------+----------------------------------------+

mysql>  select '123' union select cast(45678 as char(5));
Field   1:  `123`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_bin (46)
Length:     20
Max_length: 5
Decimals:   31
Flags:


+-------+
| 123   |
+-------+
| 123   |
| 45678 |
+-------+
2 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

release-4.0, release-5.0, release-5.1, release-5.2, release-5.3, master

[fuzhe1989]

/assign @wshwsh12

[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.