information-schema: note the extra cost of querying TABLE/PARTITION size columns

[mjonss]

What is changed, added or deleted? (Required)

Adds a Note to the INFORMATION_SCHEMA.TABLES and INFORMATION_SCHEMA.PARTITIONS reference pages explaining that querying the size columns (AVG_ROW_LENGTH, DATA_LENGTH, INDEX_LENGTH) makes TiDB read per-column size statistics from mysql.stats_histograms (one row per column of every table), which costs noticeably more time and resources on clusters with many tables than querying only TABLE_ROWS, which reads only the smaller mysql.stats_meta table.

This documents the behavior introduced by pingcap/tidb#69955 (fixing pingcap/tidb#69818), where TABLE_ROWS-only queries now skip the mysql.stats_histograms read. The note helps users avoid the size columns when they only need the estimated row count.

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):
  • Product PR: executor, statistics: skip stats_histograms read for TABLE_ROWS-only INFORMATION_SCHEMA queries tidb#69955
  • Issue: executor: avoid unnecessary stats_histograms reads for TABLE_ROWS-only queries tidb#69818

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 guidance to TABLES and PARTITIONS reference documentation about the relative cost of retrieving table size metrics.
  • Recommended selecting only TABLE_ROWS when an estimated row count is needed for better performance on clusters with many tables.

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[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between a49788b and 51e5a57.

📒 Files selected for processing (2)

• information-schema/information-schema-partitions.md
• information-schema/information-schema-tables.md

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📝 Walkthrough

Walkthrough

Changes

Information Schema Documentation

Layer / File(s)	Summary
Statistics query guidance information-schema/information-schema-partitions.md, information-schema/information-schema-tables.md	Adds notes explaining that size statistics read mysql.stats_histograms, while TABLE_ROWS reads mysql.stats_meta. Recommends selecting only TABLE_ROWS when an estimated row count is needed.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 51e5a

This localized documentation-only change adds guidance about the cost of querying size columns; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main documentation change: noting the additional cost of querying table and partition size columns.
Description check	✅ Passed	The description explains the changes, affected TiDB version, related issue and PR, AI involvement, and applicable change classifications. All required sections are present and sufficiently complete.
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.

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. (2 skipped: 2 unsupported.)

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[qiancai]

Note: if pingcap/tidb#70742 is merged in the future, we'll need to cherry-pick this doc PR to the release-8.5 branch as well.