partition by RANGE COLUMNS (varchar_bin_collation_col) not using the correct collation for checking increasing partition values

[mjonss]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a varchar(255) charset utf8mb4 collate utf8mb4_bin) partition by range columns (a) (partition pnull values less than (""), partition puppera values less than ("AAA"), partition plowera values less than ("aaa"), partition pmax values less than (MAXVALUE));

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

mysql> create table t (a varchar(255) charset utf8mb4 collate utf8mb4_bin) partition by range columns (a) (partition pnull values less than (""), partition puppera values less than ("AAA"), partition plowera values less than ("aaa"), partition pmax values less than (MAXVALUE));
Query OK, 0 rows affected (0,07 sec)

3. What did you see instead (Required)

tidb> create table t (a varchar(255) charset utf8mb4 collate utf8mb4_bin) partition by range columns (a) (partition pnull values less than (""), partition puppera values less than ("AAA"), partition plowera values less than ("aaa"), partition pmax values less than (MAXVALUE));
ERROR 1493 (HY000): VALUES LESS THAN value must be strictly increasing for each partition

4. What is your TiDB version? (Required)

Release Version: v5.5.0-alpha-442-gfbfe36ca7d
Edition: Community
Git Commit Hash: fbfe36ca7dd3a78a714872d468cebfce34e50116
Git Branch: master
UTC Build Time: 2022-03-02 11:21:12
GoVersion: go1.17
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[mjonss]

/component tablepartition

[bb7133]

The root cause is here:

tidb/ddl/ddl_api.go

Line 2685 in 5170f2f

if strings.EqualFold(curr.LessThan[i], prev.LessThan[i]) {

It gives a case-insensitive comparison between AAA and aaa, it has been introduced in a long time. IMHO it is not a big issue in most of the cases. I will lower its severity.