Wrong collation for select concat(unix_timestamp(datetime)*1000, '') from t

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql -h 127.0.0.1 -u root -P 4000 --column-type-info

mysql> create table t (dt datetime not null);
mysql> insert into t values ('2021-04-26 14:52:33');
mysql> select concat(unix_timestamp(dt)*1000, '') from t;
Field   1:  `concat(unix_timestamp(dt)*1000, '')`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  binary (63)
Length:     20
Max_length: 13
Decimals:   31
Flags:      NOT_NULL BINARY 


+--------------------------------------------------------------------------+
| concat(unix_timestamp(dt)*1000, '')                                      |
+--------------------------------------------------------------------------+
| 0x31363139343139393533303030                                             |
+--------------------------------------------------------------------------+
1 row in set (0.01 sec)

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

mysql> select concat(unix_timestamp(dt)*1000, '') from t;
Field   1:  `concat(unix_timestamp(dt)*1000, '')`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_bin (46)
Length:     80
Max_length: 13
Decimals:   31
Flags:      


+-------------------------------------+
| concat(unix_timestamp(dt)*1000, '') |
+-------------------------------------+
| 1619419953000                       |
+-------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

+--------------------------------------------------------------------------+
| concat(unix_timestamp(dt)*1000, '')                                      |
+--------------------------------------------------------------------------+
| 0x31363139343139393533303030                                             |
+--------------------------------------------------------------------------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

This bug was introduced in 4.0.11 by #19186
It also exists on 5.0 and master.

The collation of concat(cast(mul(unix_timestamp(test.t.dt), 1000), var_string(20)), '') should be utf8mb4 but get binary after this change https://github.com/pingcap/tidb/pull/19186/files#diff-3556e9d098f2adc7f39a39054ebb397bf08d9141b3f0dbba52d9f109f439f45aR162-R163

[ti-srebot]

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