executor: fix INSERT regression for tables without generated columns

[bb7133]

What problem does this PR solve?

Issue Number: close #68129

Problem Summary:

PR #67917 introduced a performance regression in INSERT statements. In fillRow() inside insert_common.go, the code unconditionally calls MutRowFromDatums(row) — which allocates a Chunk plus N *Column objects — even when the table has zero virtual generated columns (gCols is empty). This causes unnecessary heap allocations on every single inserted row, increasing GC pressure and reducing throughput.

For YCSB Workload A (mixed read/write) against an 11-column table with no generated columns (the typical usertable schema), this regression measured at −23.8% OPS compared to the pre-PR baseline.

What changed and how does it work?

Two minimal changes in pkg/executor/insert_common.go:

1. Lazy slice initialization: Changed gCols := make([]*table.Column, 0) to var gCols []*table.Column, avoiding an unnecessary heap allocation of an empty slice header on every call.

2. Early return when no generated columns: Added a fast path immediately before the MutRowFromDatums call:

   if len(gCols) == 0 {
       return row, nil
   }

   When gCols is empty (the common case for tables without virtual generated columns), we skip MutRowFromDatums entirely and return the row directly.

These changes restore pre-regression performance for the common case while leaving the generated-column path fully intact.

Check List

Tests

• Unit test (added BenchmarkInsertYCSBLike in pkg/executor/bench_gencol_test.go)
• Integration test
• Manual test (go-ycsb Workload A benchmark, 3-way comparison)
• No need to test

  • I checked and no code files have been changed.

Benchmark results (go-ycsb Workload A, 8 threads, 30 s, tidb-server --store=unistore):

Binary	OPS	vs Baseline
Baseline (f2ebab0, before #67917)	19,143	—
Regressed (35e8e04, after #67917)	14,591	−23.8%
Fixed (35e8e04 + this PR)	18,990	−0.8% (within noise)

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 performance regression in INSERT statements introduced by #67917: skip MutRowFromDatums allocation when a table has no virtual generated columns, restoring ~24% OPS throughput for workloads on plain tables.

Summary by CodeRabbit

• Refactor
  • Reduced work and allocations during INSERT when no generated columns exist by adding a fast-path that skips generated-column evaluation, improving insert performance.
• Tests
  • Added a YCSB-like benchmark to measure INSERT performance on wide tables and track regressions.

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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: 1bf76185-b484-4b13-b6f6-53b340c594c8

📥 Commits

Reviewing files that changed from the base of the PR and between 494845e and 09434ba.

📒 Files selected for processing (2)

• pkg/executor/bench_gencol_test.go
• pkg/executor/insert_common.go

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

• pkg/executor/bench_gencol_test.go
• pkg/executor/insert_common.go

---

📝 Walkthrough

Walkthrough

fillRow now builds a generated-column slice lazily and returns immediately when no generated columns exist; a new YCSB-like INSERT benchmark was added to exercise the insert path.

Changes

Generated Column Precomputation

Layer / File(s)	Summary
Data Shape / Declarations pkg/executor/insert_common.go	gCols declared as a nil slice (var) instead of a pre-initialized slice.
Core Implementation / Fast-path pkg/executor/insert_common.go	fillRow appends generated columns during the initial column iteration and returns early when len(gCols) == 0, skipping generated-column evaluation.
Benchmark pkg/executor/bench_gencol_test.go	Added BenchmarkInsertYCSBLike which creates an 11-column VARCHAR usertable and repeatedly runs INSERT IGNORE of a constant row.

Sequence Diagram(s)

(Skipped — conditions for diagram generation not met.)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• pingcap/tidb#67917: Modifies the same InsertValues.fillRow path and gencol benchmarks.

Suggested reviewers

• henrybw
• terry1purcell
• hawkingrei

Poem

🐰 I hopped through columns, one tidy scan,
Collected the generated, then filled what I can.
If none need magic, I bound past the gate—
Quick little thump, no extra work to wait.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main fix: addressing an INSERT performance regression for tables without generated columns.
Description check	✅ Passed	The description comprehensively covers the problem, the solution with code details, benchmark results, test additions (BenchmarkInsertYCSBLike), and includes a detailed release note.
Linked Issues check	✅ Passed	The PR directly addresses issue #68129 by fixing the INSERT performance regression (−23.8% OPS) caused by PR #67917, restoring throughput to near-baseline levels through optimized gCols handling.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the performance regression: optimization in insert_common.go and a targeted benchmark test in bench_gencol_test.go, with no extraneous modifications.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (2.12.1)

Command failed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.8981%. Comparing base (0e5e814) to head (09434ba).
⚠️ Report is 32 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #68187        +/-   ##
================================================
- Coverage   77.7559%   76.8981%   -0.8578%     
================================================
  Files          1990       1973        -17     
  Lines        551769     556868      +5099     
================================================
- Hits         429033     428221       -812     
- Misses       121816     128402      +6586     
+ Partials        920        245       -675     

Flag	Coverage Δ
integration	41.5131% <100.0000%> (+1.7112%)	⬆️

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	50.0597% <ø> (-13.0338%)	⬇️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, tiancaiamao

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• OWNERS [cfzjywxk,tiancaiamao]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-07 03:53:23.173499545 +0000 UTC m=+326276.046849507: ☑️ agreed by cfzjywxk.
• 2026-05-07 15:14:21.141815358 +0000 UTC m=+367134.015165340: ☑️ agreed by tiancaiamao.

[hawkingrei]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #69208.