system-variables: add tidb_paging_size_bytes (internal use)

[JmPotato]

What is changed, added or deleted? (Required)

新增系统变量 tidb_paging_size_bytes 的文档说明。

该变量由 pingcap/tidb#68091 引入，用于在 coprocessor 分页协议中，按字节数为单个 paging 响应设置上限（在按行数分页 tidb_max_paging_size 之外，额外提供按字节分页的能力）。默认值经 pingcap/tidb#69868 调整为 4194304（即 4 MiB），设为 0 表示关闭按字节数分页。该功能仅在开启资源管控且当前语句所属资源组具有固定的 RU 配额时生效，供 PD 的资源管控模块在执行语句前根据扫描数据量预估并预扣 RU。

该变量为 TiDB 内部变量，因此文档中已明确标注 不推荐修改该变量的值。

字段（作用域、是否持久化、SET_VAR Hint、类型、默认值、范围、单位）均取自源码定义（ScopeGlobal | ScopeSession、TypeUnsigned、默认 4194304、范围 [0, math.MaxInt64]）。

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

• master (the latest development version)

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

• Other reference link(s):
  • pingcap/tidb PR: *: add tidb_paging_size_bytes and forward it to coprocessor RPC tidb#68091
  • pingcap/tidb PR（默认值调整为 4 MiB）: sessionctx: default paging size bytes to 4 MiB tidb#69868
  • kvproto PR: proto: add paging_size_bytes to coprocessor.Request kvproto#1448

Summary by CodeRabbit

• 文档
  • 新增系统变量 tidb_paging_size_bytes 的说明，包括作用域、默认值、取值范围及持久化设置方式。
  • 补充按字节分页的生效条件、资源管控要求和使用限制。
  • 标注该变量为 TiDB 内部变量，不建议修改。

[qiancai]

Suggested change

	- 该变量是 TiDB 内部使用的变量，**不推荐**修改该变量的值。
	- 该变量为 TiDB 内部变量，**不推荐**修改该变量的值。

[JmPotato]

已采纳，感谢 review！

[qiancai]

Rest LGTM

[qiancai]

Suggested change

	- 这个变量用来设置 coprocessor 协议中单个 paging 响应的字节数上限，用于在按行数分页（[`tidb_max_paging_size`](#tidb_max_paging_size-从-v630-版本开始引入)）之外，额外提供按字节分页的能力。默认值为 `0`，表示关闭按字节分页。该功能仅在开启[资源管控](/tidb-resource-control-ru-groups.md)时生效，供 PD 资源管控做 RU 预扣费使用。
	- 控制 coprocessor 协议中单个分页响应的字节数上限，用于在 [`tidb_max_paging_size`](#tidb_max_paging_size-从-v630-版本开始引入) 所提供的按行数分页机制之外，额外提供按字节数分页的能力。默认值为 `0`，表示关闭按字节分页。该功能仅在开启[资源管控](/tidb-resource-control-ru-groups.md)时且当前语句所属资源组具有固定的 RU 配额时生效，供 PD 的资源管控模块在执行语句前根据扫描数据量预估并预扣 RU。

[qiancai]

reason: “预扣费”通常表示预先扣除费用，容易使人误以为 RU 是货币费用；此处实际是预扣 RU。

[JmPotato]

已采纳该措辞，并同步了 pingcap/tidb#69868 的改动：默认值更新为 4194304（即 4 MiB），同时补充说明设为 0 表示关闭按字节数分页。

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-07-14 07:23:37.163120982 +0000 UTC m=+698403.199216038: ☑️ agreed by qiancai.

[qiancai]

@JmPotato I've added a few comments on this PR. PTAL.

[ti-chi-bot]

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[coderabbitai]

📝 Walkthrough

Walkthrough

新增 tidb_paging_size_bytes 系统变量文档，说明其配置属性、默认值、取值范围、按字节数分页的生效条件，以及内部变量属性。

Changes

分页字节数变量文档

Layer / File(s)	Summary
分页字节数变量说明 system-variables.md	新增 tidb_paging_size_bytes 的版本、作用域、持久化与 Hint 设置属性、类型、默认值、范围及生效条件说明。

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

Suggested reviewers: qiancai, lilin90

🚥 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	标题准确概括了新增 tidb_paging_size_bytes 文档且强调内部使用，符合该变更。
Description check	✅ Passed	描述包含变更内容、适用版本和相关链接，且补充了默认值、作用域与内部变量说明，整体完整。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[JmPotato]

/hold