planner: do not adopt fast plan for binary literal strings (#31283)

[ti-srebot]

cherry-pick #31283 to release-5.4
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/31355

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

git push git@github.com:ti-srebot/tidb.git pr/31355:release-5.4-9cc1b169f908

---

What problem does this PR solve?

Issue Number: close #31281

Problem Summary:

mysql> create table t (a varchar(255) charset gbk primary key /*T![clustered_index] clustered */, b int);
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t values ('你好', 1);
Query OK, 1 row affected (0.00 sec)

mysql> explain select * from t where a = 0xC4E3BAC3;
+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| TableDual_2 | 0.00    | root |               | rows:0        |
+-------------+---------+------+---------------+---------------+
1 row in set (0.00 sec)

mysql> select * from t where a = 0xC4E3BAC3;
Empty set (0.00 sec)

This is because the incorrect handling for the point-get value:

tidb/planner/core/point_get_plan.go

Lines 1206 to 1222 in d9ef107

	dVal, err := d.ConvertTo(stmtCtx, &col.FieldType)
	if err != nil {
	if terror.ErrorEqual(types.ErrOverflow, err) {
	return append(nvPairs, nameValuePair{colName: colName.Name.Name.L, value: d, param: param}), true
	}
	// Some scenarios cast to int with error, but we may use this value in point get.
	if !terror.ErrorEqual(types.ErrTruncatedWrongVal, err) {
	return nil, false
	}
	}
	// The converted result must be same as original datum.
	cmp, err := dVal.Compare(stmtCtx, &d, collate.GetCollator(col.Collate))
	if err != nil {
	return nil, false
	} else if cmp != 0 {
	return append(nvPairs, nameValuePair{colName: colName.Name.Name.L, value: dVal, param: param}), true
	}

After #31031, ConvertTo() takes the responsibility of converting the binary literal from a given charset to utf-8 strings. In this code snippet, line 1220 assumes that all the values processed by ConvertTo() should be identical to themself. Otherwise, this query gets nothing.

This assumption may be correct for truncating case like querying "aaaa" against char(2) column, but it is not true for binary literal strings.

What is changed and how it works?

In this PR, I remove the above assumption and let it fallback to normal plan building. While it may affect performance, they are corner cases anyway.

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• crazycs520
• zimulala

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-srebot]

/run-all-tests

[ti-srebot]

@tangenta you're already a collaborator in bot's repo.

[zimulala]

LGTM

[tangenta]

@pingcap/qa-release-merge PTAL

[VelocityLight]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: cc1d538

[VelocityLight]

/merge

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.