Wrong behavior when handle strcmp with default collation

[solotzg]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (a varchar(100)) CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
alter table t set tiflash replica 1;

insert into t values('1   '), ('1\n'), ('1');
select hex(min(a)) from t;

MySQL version

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.29    |
+-----------+
1 row in set (0.00 sec)

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

MySQL [test]> select hex(min(a)) from t;
+-------------+
| hex(min(a)) |
+-------------+
| 31202020    |
+-------------+

3. What did you see instead (Required)

MySQL [test]> select hex(min(a)) from t;
+-------------+
| hex(min(a)) |
+-------------+
| 31          |
+-------------+

4. What is your TiFlash version? (Required)

master

TiDB behavior

For mysql

mysql> SET NAMES utf8mb4 COLLATE utf8mb4_bin;
Query OK, 0 rows affected (0.00 sec)

mysql> select strcmp('1\0', '1');
+--------------------+
| strcmp('1\0', '1') |
+--------------------+
|                 -1 |
+--------------------+
1 row in set (0.00 sec)

For tidb

MySQL [test]> SET NAMES utf8mb4 COLLATE utf8mb4_bin;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> select strcmp('1\0', '1');
+--------------------+
| strcmp('1\0', '1') |
+--------------------+
|                  1 |
+--------------------+
1 row in set (0.00 sec)

MySQL cmp '1\0' and '1':

• get min length of both string is 1;
• '1' and '1' are equal;
• the remain str in '1\0' is '\0', cmp '\0' with whit space ' ';
• cmp '\0' and ' ', same like cmp 0x0 and 0x20, got -1;

---

According to https://docs.pingcap.com/tidb/dev/character-set-and-collation, the tidb choose to remove tail space.

Bug in tiflash

tiflash/dbms/src/Columns/ColumnString.cpp

Lines 317 to 318 in 432c153

	reinterpret_cast<const char *>(&parent.chars[parent.offsetAt(lhs)]), parent.sizeAt(lhs),
	reinterpret_cast<const char *>(&parent.chars[parent.offsetAt(rhs)]), parent.sizeAt(rhs));

tiflash show remove tail '\0'.