orderby clause ref priority doesn't take effect

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `x` (
  `a` varchar(20) COLLATE utf8mb4_bin NOT NULL,
  `b` decimal(10,4) DEFAULT NULL,
  PRIMARY KEY (`a`),
  UNIQUE KEY `b` (`b`)
) COLLATE=utf8mb4_bin

mysql> select * from x;
+---+---------+
| a | b       |
+---+---------+
| i | -1.0000 |
| I |  1.0988 |
+---+---------+
2 rows in set (0.03 sec)

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

mysql

mysql> select a, char_length(b) from x order by lower(a), b;
+---+----------------+
| a | char_length(b) |
+---+----------------+
| i |              7 |
| I |              6 |
+---+----------------+
2 rows in set (0.02 sec)

mysql> select a, char_length(b) as b from x order by lower(a), b;
+---+------+
| a | b    |
+---+------+
| I |    6 |
| i |    7 |
+---+------+
2 rows in set (0.03 sec)

3. What did you see instead (Required)

tidb

tidb> select a, char_length(b)  from x order by lower(a), b;
+---+----------------+
| a | char_length(b) |
+---+----------------+
| i |              7 |
| I |              6 |
+---+----------------+
2 rows in set (0.00 sec)

tidb> select a, char_length(b)  as b from x order by lower(a), b;
+---+------+
| a | b    |
+---+------+
| i |    7 |
| I |    6 |
+---+------+
2 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

git hash: 4220dfa

[AilinKid]

note that: it has been fixed implicitly by: https://github.com/pingcap/tidb/pull/45651/files#diff-3b47fd3bf2c33ae3a03037a65ed39df33a4ae73debe9cf6f5bb2f397508061a0R2819