expression: avoid padding 0 when implicitly cast to binary (#35053)

[ti-srebot]

cherry-pick #35053 to release-6.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/35558

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/35558:release-6.0-49603af35971

---

What problem does this PR solve?

Issue Number: close #34823

Problem Summary:

For between and expression, implicitly casting string charset to binary string will pad 0 to the origin string, which causes different string comparison results from mysql. We can instead use mysql.TypeVarStringto avoid this problem. In order to match the behavior of mysql, we can instead avoid doing implicit casting here. See String::needs_conversion() in mysql-server for details.

Modification rationale:

For one thing, we still need to build cast function to retain binary collation information till execution phase. For another, we can not keep using fixed length representation otherwise 0 padding will affect later string comparison result. Therefore, we need to use VarString type of cast here.

drop table if exists t1;drop table if exists t2;
create table t1(a char(20));
create table t2(b binary(20), c binary(20));
insert into t1 value('-1');
insert into t2 value(0x2D31, 0x67);
insert into t2 value(0x2D31, 0x73);
select * from t1, t2 where t1.a between t2.b and t2.c;

// previously

mysql> select * from t1, t2 where t1.a between t2.b and t2.c;
+------+--------------------------------------------+--------------------------------------------+
| a    | b                                          | c                                          |
+------+--------------------------------------------+--------------------------------------------+
| -1   | 0x2D31000000000000000000000000000000000000 | 0x7300000000000000000000000000000000000000 |
| -1   | 0x2D31000000000000000000000000000000000000 | 0x6700000000000000000000000000000000000000 |
+------+--------------------------------------------+--------------------------------------------+
2 rows in set (0.06 sec)

// now

mysql> select * from t1, t2 where t1.a between t2.b and t2.c;
Empty set (0.01 sec)

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
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[ti-srebot]

/run-all-tests