expression: clone constant before PropagateType mutation

[hawkingrei]

What problem does this PR solve?

Issue Number: close #66706

Problem Summary:

When a view projects a DECIMAL constant and an outer predicate applies SIGN() to that view column, the implicit cast-to-real path can propagate widened DECIMAL metadata back into the shared constant. The projected view column can then be displayed with a widened scale, for example 0.990000000000000000000000000000 instead of 0.99.

What changed and how does it work?

PropagateType now clones *expression.Constant before mutating propagated type metadata, matching the existing clone-before-mutate handling for Column and CorrelatedColumn. This keeps the cast-local widened metadata from leaking back into a shared view projection constant.

This PR also adds:

• a focused pkg/expression unit assertion that the original constant keeps DECIMAL(5,2) while the cast-local clone gets the propagated type;
• a planner issue regression for the direct view predicate and derived-table control query, run under both default and cascades planner modes;
• test case map updates for the touched test files.

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.

Manual test:

go test ./.tmp/issue66706 -run TestIssue66706Replay -count=1 -tags=intest,deadlock -v

Before the fix, the direct view-predicate query returned 0.990000000000000000000000000000 with result field DECIMAL(48,30), while the derived-table control returned 0.99. After the fix, both returned 0.99 and the direct result field stayed DECIMAL(5,2).

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 an issue that a DECIMAL constant projected from a view could be displayed with a widened scale when the view column was also used in a SIGN() predicate.

Summary by CodeRabbit

• Bug Fixes

  • Constants are no longer mutated during numeric type propagation, preventing loss of scale/precision (fixes cases like SIGN() on view columns).

• Tests

  • Added unit and regression tests to verify decimal constant handling, ensuring propagated expressions preserve numeric metadata and preventing regressions.

[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: 783b865d-77dc-4e52-9ba3-eb1383a9219c

📥 Commits

Reviewing files that changed from the base of the PR and between f4d2d8b and f3ff867.

📒 Files selected for processing (3)

• pkg/expression/expression.go
• pkg/expression/expression_test.go
• pkg/planner/core/issuetest/planner_issue_test.go

🚧 Files skipped from review as they are similar to previous changes (2)

• pkg/expression/expression_test.go
• pkg/planner/core/issuetest/planner_issue_test.go

---

📝 Walkthrough

Walkthrough

Clone Constant nodes during ETReal type propagation to avoid mutating decimal metadata; add unit test and cascades regression test validating SIGN() on view-returned decimals preserves scale and emits no warnings.

Changes

Decimal scale leak fix in type propagation

Layer / File(s)	Summary
Constant cloning in type propagation and unit test pkg/expression/expression.go, pkg/expression/expression_test.go	PropagateType now clones *Constant arguments in the types.ETReal branch before adjusting type metadata. TestConstant adds coverage for decimal constants, asserting the propagated argument is a cloned instance with expected Flen/Decimal metadata while the original RetType remains unchanged.
Regression test for issue-66706 pkg/planner/core/issuetest/planner_issue_test.go	New cascades regression test issue-66706-decimal-scale-leak-through-sign-view-predicate verifies SELECT v0.c0 FROM v0 WHERE SIGN(v0.c0) returns 0.99 and produces no warnings, and repeats the check with a derived-table form.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• pingcap/tidb#66652: Related work handling deep-copying/ownership of FieldType/RetType for constants and planner-built expressions to avoid shared metadata mutation.

Suggested reviewers

• winoros
• qw4990
• guo-shaoge

Poem

🐇 I cloned the constant, kept its scale bright,
No creeping zeros in the moonlit night.
SIGN() on views now sings true and small,
Precision preserved — rejoice, one and all! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% 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: clone constant before PropagateType mutation' clearly and specifically describes the main code change, matching the PR objectives and the core fix in pkg/expression/expression.go.
Description check	✅ Passed	The description includes all required template sections: Issue Number (close #66706), Problem Summary, What Changed explanation, comprehensive Check List (unit test checked, manual test provided), Side effects assessment, Documentation (affects user behaviors and MySQL compatibility checked), and a well-formatted Release note.
Linked Issues check	✅ Passed	The PR directly addresses all objectives from issue #66706 by cloning *expression.Constant before PropagateType mutation to prevent scale widening, adding unit tests validating constant preservation, planner regression tests covering both direct view predicates and derived-table equivalence, and provides a release note documenting the fix.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to issue #66706: the core fix in pkg/expression/expression.go clones constants during type propagation, the unit test verifies constant preservation during cast, and the planner regression test reproduces the original bug scenario. No unrelated changes detected.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.7084%. Comparing base (a62a6d1) to head (f3ff867).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #68433        +/-   ##
================================================
- Coverage   77.2798%   75.7084%   -1.5715%     
================================================
  Files          2010       2008         -2     
  Lines        555326     563442      +8116     
================================================
- Hits         429155     426573      -2582     
- Misses       125251     136812     +11561     
+ Partials        920         57       -863     

Flag	Coverage Δ
integration	41.5270% <ø> (+1.7340%)	⬆️

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

Components	Coverage Δ
dumpling	60.4679% <ø> (ø)
parser	∅ <ø> (∅)
br	49.9534% <ø> (-13.0557%)	⬇️

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[hawkingrei]

/retest-required

[windtalker]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guo-shaoge, windtalker

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

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Details

Needs approval from an approver in each of these files:

• pkg/expression/OWNERS [windtalker]
• pkg/planner/OWNERS [guo-shaoge]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-19 03:31:11.069116422 +0000 UTC m=+235000.573247097: ☑️ agreed by windtalker.
• 2026-05-19 03:32:59.827437196 +0000 UTC m=+235109.331567872: ☑️ agreed by guo-shaoge.

[hawkingrei]

/retest