expression: fix isBinCollation to recognize utf8mb4_0900_bin

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #68845

Problem Summary:

CONCAT_WS over a utf8mb4_0900_bin column + a utf8mb4_unicode_ci column + a mediumblob column fails with ERROR 3854 (HY000): Cannot convert string from binary to utf8mb4. The root cause is that isBinCollation() in pkg/expression/collation.go does not recognize utf8mb4_0900_bin as a binary collation, causing inferCollation to degrade to CoercibilityNone instead of keeping the _bin collation. This leads to an incorrect from_binary() cast on the blob.

What changed and how does it work?

1. Added CollationUTF8MB40900Bin = "utf8mb4_0900_bin" constant to pkg/parser/charset/charset.go.
2. Added charset.CollationUTF8MB40900Bin to the isBinCollation() check in pkg/expression/collation.go.
3. Added 3 regression test cases in pkg/expression/collation_test.go:
   • utf8mb4_0900_bin vs utf8mb4_unicode_ci (both orders) → _bin wins
   • Three-column case (utf8mb4_0900_bin + utf8mb4_unicode_ci + binary) → binary wins without from_binary() cast

Check List

Tests

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

  • I checked and no code files have been changed.

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

Fix a bug where CONCAT_WS with utf8mb4_0900_bin and a blob column incorrectly returned ERROR 3854 instead of returning binary result

Summary by CodeRabbit

• Bug Fixes

  • Improved collation handling to properly recognize utf8mb4_0900_bin as a binary collation, ensuring correct collation inference when mixed with other collations.

• Tests

  • Added regression test coverage for utf8mb4_0900_bin collation handling scenarios.

• Documentation

  • Clarified binary-collation recognition behavior to reduce ambiguity in collation conflict resolution.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 6b4cdabf-7c30-4701-a18c-383d22a01a01

📥 Commits

Reviewing files that changed from the base of the PR and between 1ce5433 and 593e5ed.

📒 Files selected for processing (2)

• pkg/expression/collation.go
• pkg/util/collate/collate.go

🚧 Files skipped from review as they are similar to previous changes (1)

• pkg/expression/collation.go

---

📝 Walkthrough

Walkthrough

Adds an exported constant for utf8mb4_0900_bin, treats that collation as binary in both utility and expression-level predicates, and adds regression tests ensuring correct collation inference when mixed with non-UTF8 blobs.

Changes

UTF8MB4 0900 Binary Collation Support

Layer / File(s)	Summary
Collation constant definition pkg/parser/charset/charset.go	New exported constant CollationUTF8MB40900Bin = "utf8mb4_0900_bin".
IsBinCollation update pkg/util/collate/collate.go	IsBinCollation doc and predicate updated to use the charset constant and clarify included/excluded collations.
Expression-level binary check pkg/expression/collation.go	isBinCollation extended to recognize CollationUTF8MB40900Bin, affecting collation derivation logic.
Collation inference test coverage pkg/expression/collation_test.go	Three new cases in TestInferCollation validate selection of utf8mb4_0900_bin over non-binary collations and binary handling with blob values.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

size/S, ok-to-test

Suggested reviewers

• qw4990
• hawkingrei
• AilinKid

Poem

🐰 I found a bin that hid from sight,
I added a name, gave it its right,
Now mixed blobs and 0900 sing,
No more errors when they bring,
A tidy, happy charset night.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the primary change: adding utf8mb4_0900_bin recognition to the isBinCollation function to fix a specific bug.
Linked Issues check	✅ Passed	The PR addresses all coding objectives from issue #68845: added CollationUTF8MB40900Bin constant, updated isBinCollation to recognize utf8mb4_0900_bin, and added comprehensive regression tests covering the bug scenario.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the isBinCollation recognition of utf8mb4_0900_bin; no unrelated modifications are present.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-0900-bin-collation

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.3104%. Comparing base (5a38505) to head (593e5ed).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #68846        +/-   ##
================================================
- Coverage   76.3105%   75.3104%   -1.0001%     
================================================
  Files          2041       2025        -16     
  Lines        563393     568815      +5422     
================================================
- Hits         429928     428377      -1551     
- Misses       132549     140347      +7798     
+ Partials        916         91       -825     

Flag	Coverage Δ
integration	41.2874% <100.0000%> (+1.5089%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	60.4610% <ø> (ø)
parser	∅ <ø> (∅)
br	49.8023% <ø> (-13.0287%)	⬇️

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[tiancaiamao]

/retest

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[tiancaiamao]

/retest

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[tiancaiamao]

/retest

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