add the release notes for v8.5.8 (#23522)

[ti-chi-bot]

This is an automated cherry-pick of #23522

What is changed, added or deleted? (Required)

As in the title

Which TiDB version(s) do your changes apply to? (Required)

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• Documentation

  • Added TiDB 8.5.8 release notes, including optimizer, TiCDC, and component bug fixes.
  • Updated the release index, table of contents, and timeline with the August 27, 2026 release.
  • Updated documented version information to TiDB 8.5.8 and added the release date.
  • Expanded upgrade guidance to reference recent 8.5 releases.

• Bug Fixes

  • Removed unresolved merge-conflict content from the upgrade documentation.

[ti-chi-bot]

@qiancai This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The PR adds TiDB 8.5.8 release notes dated 2026-08-27, updates release indexes and version metadata, and modifies TiUP upgrade documentation.

Changes

TiDB 8.5.8 release

Layer / File(s)	Summary
Release notes content releases/release-8.5.8.md	Adds release metadata, five improvements, and bug fixes for TiDB, TiKV, PD, TiFlash, Backup & Restore, TiCDC, and TiDB Lightning.
Release index and timeline TOC-tidb-releases.md, releases/_index.md, releases/release-timeline.md	Adds TiDB 8.5.8 links and the 2026-08-27 release date to the release navigation and timeline.
Version metadata and TiUP instructions variables.json, upgrade-tidb-using-tiup.md	Updates the TiDB version and release date. The TiUP documentation contains unresolved merge-conflict markers and competing release-note sections.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to 5f07e

The release documentation change still contains unresolved merge conflicts: the metadata file is invalid and the upgrade instructions are contradictory. These issues can break documentation processing and mislead users, so they must be resolved before merge.

Suggested reviewers: qiancai

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description includes the required sections, but the change summary is vague, no TiDB version checkbox is selected, and the reference-link fields are empty.	Describe the release-note files and changes directly, select the applicable version checkbox (master, according to the PR objectives), and provide the related PR or file links. Also confirm whether the unchecked optional items apply.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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 clearly identifies the main change: adding the TiDB v8.5.8 release notes.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (6 skipped: 6 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 96c0d896-4b51-4003-9d0d-027e5e930d36

📥 Commits

Reviewing files that changed from the base of the PR and between ff98075 and 5f07eec.

📒 Files selected for processing (6)

• TOC-tidb-releases.md
• releases/_index.md
• releases/release-8.5.8.md
• releases/release-timeline.md
• upgrade-tidb-using-tiup.md
• variables.json

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

[ti-chi-bot]

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