sessionctx: restore tidb_paging_size_bytes default to 0

[JmPotato]

What problem does this PR solve?

Issue Number: ref #68085

Problem Summary:

#69868 changed the default of tidb_paging_size_bytes from 0 to 4 MiB. After further consideration, keep byte-budget paging opt-in by default to avoid surprising behavior changes for clusters that have not explicitly enabled this path. Operators can still set a non-zero value when needed.

What changed and how does it work?

• Restore DefPagingSizeBytes / tidb_paging_size_bytes default from 4 MiB back to 0 (disabled).
• Keep the comments that document:
  • when a non-zero value takes effect (Resource Control enabled and the active Resource Group is non-burstable);
  • the tested workload matrix and why 4 MiB is a good candidate when enabling the feature.
• Update integration-test expectations for the initial and reset-to-default values.

This intentionally does not remove the SET_VAR hint support or the
scope documentation introduced around #69868.

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.

Restore the default of `tidb_paging_size_bytes` to `0` (byte-budget paging disabled). Set a non-zero value such as 4 MiB (4194304) to enable it for resource groups with limited burst.

Summary by CodeRabbit

• Bug Fixes
  • Changed the default value of tidb_paging_size_bytes to 0, disabling byte-budget paging by default.
  • Byte-budget paging now applies only when Resource Control is enabled and the active resource group is non-burstable.

[ti-chi-bot]

@YuhaoZhang00: adding LGTM is restricted to approvers and reviewers in OWNERS files.

Details

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[coderabbitai]

📝 Walkthrough

Walkthrough

The default for tidb_paging_size_bytes changes from 4 MiB to 0, disabling byte-budget paging by default. Integration test expectations are updated for both default-reset checks.

Changes

Paging Default

Layer / File(s)	Summary
Paging default and validation pkg/sessionctx/vardef/tidb_vars.go, tests/integrationtest/r/sessionctx/setvar.result	Sets DefPagingSizeBytes to 0, updates its documentation, and changes reset-value expectations from 4194304 to 0.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• pingcap/tidb#68091: Introduces and propagates tidb_paging_size_bytes, which this change updates.
• pingcap/tidb#69633: Adjusts when paging is forced to zero based on resource-group conditions.
• pingcap/tidb#69868: Modifies the same variable and integration test expectations.

Suggested reviewers: qw4990, guo-shaoge, yudongusa, yuhaozhang00

Poem

I’m a rabbit hopping through the bytes,
Paging sleeps beneath moonlit nights.
Four MiB fades into zero,
Tests now cheer, “That’s the hero!”
🐇 Defaults rest where budgets cease.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: restoring the default of tidb_paging_size_bytes to 0.
Description check	✅ Passed	The description matches the template with issue reference, problem summary, change details, checklist items, side effects, docs, and release note.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.9390%. Comparing base (0b84b15) to head (97cbc20).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #70052        +/-   ##
================================================
- Coverage   76.3266%   73.9390%   -2.3877%     
================================================
  Files          2041       2058        +17     
  Lines        559579     578881     +19302     
================================================
+ Hits         427108     428019       +911     
- Misses       131570     150506     +18936     
+ Partials        901        356       -545     

Flag	Coverage Δ
integration	40.9763% <ø> (+1.2710%)	⬆️

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

Components	Coverage Δ
dumpling	59.8807% <ø> (ø)
parser	∅ <ø> (∅)
br	47.4060% <ø> (-15.3154%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[yudongusa]

Please open a document PR on this.

[JmPotato]

Please open a document PR on this.

Please take a look at pingcap/docs-cn#21752.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rleungx, yudongusa, YuhaoZhang00

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

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [rleungx]
• pkg/sessionctx/vardef/OWNERS [yudongusa]

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:

• 2026-07-24 15:06:08.175444212 +0000 UTC m=+1590154.211539288: ☑️ agreed by yudongusa.
• 2026-07-27 05:14:04.80265169 +0000 UTC m=+1813830.838746756: ☑️ agreed by rleungx.

[JmPotato]

/cherry-pick release-nextgen-202603

[ti-chi-bot]

@JmPotato: new pull request created to branch release-nextgen-202603: #70081.

Details

In response to this:

/cherry-pick release-nextgen-202603

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.