[lohmag]

What problem does this PR solve?

Issue Number: close #69823

Problem Summary:

RAND(N) with a constant seed creates its generator once, when the expression is built. A pessimistic statement retried after a lock error reuses the plan and its expressions, so the retry continues the sequence of the failed attempt instead of starting it again. For example UPDATE t SET u = IF(RAND(12345) < 0.8, 1, 2) writes u = 2 after a retry, where a single execution gets u = 1 and a duplicate key error.

What changed and how does it work?

• RAND(N) with a constant seed keeps the seed and creates its generator per evaluation context (builtinFuncCache, keyed by the context ID), so it still generates one sequence for all the rows of an execution.

Visible change: a prepared SELECT RAND(N) served from the plan cache used to continue one sequence across executions. It now starts from the seed on every execution, as a fresh execution and MySQL do. Each clone of a seeded RAND for the instance plan cache gets its own generator cache, so concurrent executions of a cached plan no longer restart each other's sequence. A seeded RAND in an index hash join's other condition, which each worker clones, restarts the sequence per worker. The order of its values across workers was not deterministic before either.

• StatementContext.ResetForRetry gives the statement context a new context ID, so the retried attempt starts the sequence again. The other expressions that cache per context ID only cache values derived from constants, which they rebuild.

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.

TestPessimisticRetryRestartsSeededRand fails without the fix and passes with it. pkg/expression, pkg/executor/test/txn, pkg/sessiontxn/..., pkg/executor and the integration tests expression/builtin and select pass, except tests that also fail on master in my environment.

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 a retried statement in a pessimistic transaction might get different values from `RAND(N)` with a constant seed than a single execution.

Summary by CodeRabbit

• Bug Fixes
  • Seeded RAND() expressions now behave correctly when a statement is retried, helping ensure retries produce the expected result.
  • Fixed an issue where a pessimistic transaction retry could fail to report a duplicate-key error after a competing transaction committed.

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[coderabbitai]

Walkthrough

Statement retries receive a new context ID. Seeded RAND generators are cached per evaluation context and used by scalar and vector evaluation. A transaction test checks the pessimistic retry result, and the test target’s shard count increases.

Changes: Seeded RAND retry behavior

Layer / File(s)	Summary
Retry-scoped seeded RAND evaluation pkg/sessionctx/stmtctx/stmtctx.go, pkg/expression/builtin_math.go, pkg/expression/builtin_math_vec.go, pkg/executor/test/txn/txn_test.go, pkg/executor/test/txn/BUILD.bazel	ResetForRetry assigns a new context ID. Seeded RAND obtains a generator cached for the evaluation context in scalar and vector paths. The transaction test checks for a duplicate-key error, a positive retry count, and unchanged rows. The Bazel target’s shard count changes from 11 to 12.

Priority: Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix · Severity of issue fixed: Medium

Possibly related PRs

• pingcap/tidb#72274: Both changes address state from failed attempts affecting transparent pessimistic retries.

Suggested reviewers: terry1purcell

Merge Risk: Moderate · up to 86a1f

Seeded RAND can repeat values during interleaved evaluations, producing results inconsistent with one uninterrupted execution. Give clones independent caches before merging unless this risk is explicitly accepted.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title is concise, specific, and accurately describes the main change: restarting seeded RAND evaluation when a pessimistic statement is retried.
Linked Issues check		Issue #69823 has one direct coding requirement. A constant-seed RAND(12345) must not advance past values consumed by a failed pessimistic attempt. The retry must expose the duplicate-key conflict and …
Out of Scope Changes check		All changed files relate to the seeded RAND retry requirement of #69823: - pkg/expression/builtin_math.go and builtin_math_vec.go: change how constant-seed RAND obtains its generator. The vectorized p…
Description check		The pull request description follows the repository template. It includes the issue number, problem summary, implementation details, test coverage, side effects, documentation impact, and a release no…

Full details: Linked Issues check

Explanation

Issue #69823 has one direct coding requirement. A constant-seed RAND(12345) must not advance past values consumed by a failed pessimistic attempt. The retry must expose the duplicate-key conflict and keep row 1 unchanged.

Evidence from the reviewed diff (merge base 0073665..., head 86a1f25...):

• pkg/expression/builtin_math.go: For a constant seed, randFunctionClass.getFunction now stores the seed and a builtinFuncCache[*mathutil.MysqlRng] instead of a single pre-built MysqlRng. builtinRandSig.getRng creates the generator from the seed per evaluation context. evalReal and vecEvalReal use getRng. Clone copies the seed and cache reference. The unseeded RAND() path still uses ctx.Rng().
• pkg/sessionctx/stmtctx/stmtctx.go: ResetForRetry generates a new context ID. The next attempt therefore starts the seeded sequence again, so its first value is the value a fresh execution gets (u=1, duplicate key).
• Per the raw summary, pkg/executor/test/txn/txn_test.go adds TestPessimisticRetryRestartsSeededRand. It checks for a duplicate-key error after a competing commit, an incremented retry count, and unchanged rows. Per the PR description, it fails without the fix and passes with it.

The approach restores the generator state from a fresh context for each attempt. It does not only deep-copy the RNG after the first attempt, which the issue calls too late.

Limits of this review: I did not read the body of builtinFuncCache.getOrInitCache, and I did not read the test source directly. The mechanism relies on that helper rebuilding its value when the context ID changes. The PR description and the test summary support this behavior, but I did not confirm it in source. I could not run the tests.

Full details: Out of Scope Changes check

Explanation

All changed files relate to the seeded RAND retry requirement of #69823:

• pkg/expression/builtin_math.go and builtin_math_vec.go: change how constant-seed RAND obtains its generator. The vectorized path uses the same getRng helper, so the scalar and vector paths behave the same.
• pkg/sessionctx/stmtctx/stmtctx.go: the new context ID on ResetForRetry is the mechanism that restarts the sequence.
• pkg/executor/test/txn/txn_test.go and BUILD.bazel: add the regression test and raise txn_test shard_count from 11 to 12 to include it.

No change is unrelated to the requirement. The ResetForRetry change affects every expression that caches state by context ID, not only RAND. The PR description states that those caches hold only constant-derived values. I did not verify that claim in this review. It is a side-effect risk, not a demonstrated out-of-scope change.

Full details: Description check

Explanation

The pull request description follows the repository template. It includes the issue number, problem summary, implementation details, test coverage, side effects, documentation impact, and a release note.

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_{A rabbit checks the seeded stream,
New contexts keep each retry’s beam.
The rows stay still when conflicts bite,
The tests record the outcome right.
Then hops away beneath moonlight.}

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Hi @lohmag. Thanks for your PR.

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[coderabbitai]

Actionable comments posted: 1

---

• 🪄 Fix comments: Fix CodeRabbit comments on this PR

Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @pkg/expression/builtin_math.go:
- Line 1206: Update builtinRandSig.Clone to avoid sharing the seededRng cache
pointer with the original expression. When the original has a seededRng cache,
give the clone its own empty cache; preserve nil when no cache exists and keep
copying mysqlRng and seed.

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Commits

Reviewing files that changed from the base of the PR and between 0073665 and 86a1f25.

Files selected for processing: 5

• pkg/executor/test/txn/BUILD.bazel
• pkg/executor/test/txn/txn_test.go
• pkg/expression/builtin_math.go
• pkg/expression/builtin_math_vec.go
• pkg/sessionctx/stmtctx/stmtctx.go

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