information-schema: note the extra cost of querying TABLE/PARTITION size columns by mjonss · Pull Request #23685 · pingcap/docs

mjonss · 2026-09-01T05:13:19Z

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.

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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

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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.

…ize columns Add a note to the TABLES and PARTITIONS pages explaining that AVG_ROW_LENGTH, DATA_LENGTH, and INDEX_LENGTH read per-column size statistics from mysql.stats_histograms and cost more than querying only TABLE_ROWS, which reads only mysql.stats_meta. Documents the behavior from pingcap/tidb#69818. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ti-chi-bot · 2026-09-01T05:13:27Z

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coderabbitai · 2026-09-01T05:14:23Z

No actionable comments were generated in the recent review. 🎉

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

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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.

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qiancai · 2026-09-03T06:41:37Z

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.

qiancai · 2026-09-03T06:35:39Z


+> **Note:**
+>
+> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.


Suggested change

> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

> Querying `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read column-size statistics from `mysql.stats_histograms`. On clusters with a large number of tables, selecting any of these columns can consume more time and resources than selecting only `TABLE_ROWS`, which only requires row-count statistics from `mysql.stats_meta`. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

reason: to focus on the user-visible performance impact. The exact layout of mysql.stats_histograms is an implementation detail, and saying it holds one row for each column of every table is too absolute because the table also stores index statistics.

qiancai · 2026-09-03T06:38:56Z


+> **Note:**
+>
+> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.


Suggested change

> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

> Querying `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read column-size statistics from `mysql.stats_histograms`. On clusters with a large number of tables, selecting any of these columns can consume more time and resources than selecting only `TABLE_ROWS`, which only requires row-count statistics from `mysql.stats_meta`. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

ti-chi-bot Bot added missing-translation-status This PR does not have translation status info. size/XS Denotes a PR that changes 0-9 lines, ignoring generated files. labels Sep 1, 2026

qiancai self-assigned this Sep 3, 2026

qiancai added the translation/doing This PR's assignee is translating this PR. label Sep 3, 2026

ti-chi-bot Bot removed the missing-translation-status This PR does not have translation status info. label Sep 3, 2026

qiancai added missing-translation-status This PR does not have translation status info. v9.0-beta.3 This PR/issue applies to TiDB v9.0-beta.3. labels Sep 3, 2026

qiancai reviewed Sep 15, 2026

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information-schema: note the extra cost of querying TABLE/PARTITION size columns - #23685

information-schema: note the extra cost of querying TABLE/PARTITION size columns#23685
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mjonss:table-rows-only-stats-cost-69818

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