expression: fix WEEK NULL mode handling

[hawkingrei]

What problem does this PR solve?

Issue Number: close #59444

Problem Summary:

WEEK('2023-01-01', NULL) returns NULL in TiDB, while MySQL returns the same result as mode 0.

What changed and how does it work?

This PR treats a NULL second argument to WEEK(date, mode) as mode 0, matching the existing YEARWEEK(date, mode) behavior and MySQL compatibility.

The change is local to WEEK expression evaluation:

• update scalar builtinWeekWithModeSig
• update vectorized builtinWeekWithModeSig
• add scalar, vectorized, and SQL-level regression coverage

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

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

Fix the issue that WEEK(date, NULL) returns NULL instead of using week mode 0.

Summary by CodeRabbit

Bug Fixes

• Fixed the WEEK() function to properly handle NULL mode arguments by defaulting to mode 0 instead of returning NULL for the entire expression, achieving MySQL compatibility.

[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: 5aadb7c2-c645-4997-bb39-57ab6405c7c9

📥 Commits

Reviewing files that changed from the base of the PR and between 6a6eefe and 39d7354.

📒 Files selected for processing (5)

• pkg/expression/builtin_time.go
• pkg/expression/builtin_time_test.go
• pkg/expression/builtin_time_vec.go
• pkg/expression/builtin_time_vec_test.go
• pkg/expression/integration_test/integration_test.go

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📝 Walkthrough

Walkthrough

The PR fixes the WEEK() builtin function to align with MySQL behavior: when the optional mode argument is NULL, it now defaults to mode 0 and returns a computed week number instead of NULL. The fix is applied to both scalar and vectorized evaluation code paths, with corresponding unit and integration test coverage.

Changes

WEEK() NULL mode default

Layer / File(s)	Summary
Scalar WEEK evaluation with NULL mode handling pkg/expression/builtin_time.go, pkg/expression/builtin_time_test.go	builtinWeekWithModeSig.evalInt now distinguishes NULL from errors: when mode is NULL, it assigns mode to 0 instead of returning NULL for the entire expression. Unit test confirms WEEK("2023-01-01", nil) returns week number 1.
Vectorized WEEK evaluation with NULL mode handling pkg/expression/builtin_time_vec.go, pkg/expression/builtin_time_vec_test.go	builtinWeekWithModeSig.vecEvalInt is updated to resolve NULL week-mode input to 0 instead of marking the output row as NULL. Vectorized test adds explicit NULL mode argument case for the (ETDatetime, ETInt) signature.
Integration test for WEEK NULL mode pkg/expression/integration_test/integration_test.go	End-to-end verification added: week("2023-01-01", null) returns 1 with no warnings.

🎯 2 (Simple) | ⏱️ ~8 minutes

🐰 A week that's null becomes a week that's one,
Default modes make the function complete and run,
MySQL whispers "zero is the key,"
Scalar and vector now in harmony! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% 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
Title check	✅ Passed	The title 'expression: fix WEEK NULL mode handling' clearly and concisely identifies the main change: fixing how the WEEK function handles NULL mode arguments.
Description check	✅ Passed	The PR description follows the repository template with all required sections: issue number, problem summary, what changed, comprehensive test coverage, side effects, and release notes.
Linked Issues check	✅ Passed	The PR fully addresses the requirements in issue #59444 by implementing WEEK(date, NULL) to return the same result as WEEK(date, 0), matching MySQL behavior, with comprehensive test coverage.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing WEEK NULL mode handling in both scalar and vectorized implementations, with corresponding test coverage and no unrelated modifications.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.7280%. Comparing base (6a6eefe) to head (39d7354).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #68440        +/-   ##
================================================
- Coverage   77.2762%   75.7280%   -1.5483%     
================================================
  Files          2010       2008         -2     
  Lines        555477     563839      +8362     
================================================
- Hits         429252     426984      -2268     
- Misses       125305     136720     +11415     
+ Partials        920        135       -785     

Flag	Coverage Δ
integration	41.1239% <0.0000%> (+1.3299%)	⬆️

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

Components	Coverage Δ
dumpling	60.4888% <ø> (ø)
parser	∅ <ø> (∅)
br	49.9725% <ø> (-13.0354%)	⬇️

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[hawkingrei]

/retest-required

[windtalker]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qw4990, windtalker

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• pkg/expression/OWNERS [windtalker]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-18 08:12:21.989902566 +0000 UTC m=+165471.494033232: ☑️ agreed by windtalker.
• 2026-05-18 12:51:35.907022942 +0000 UTC m=+182225.411153608: ☑️ agreed by qw4990.