system-variables: add optimizer cost factors

[terry1purcell]

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• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Documented all of the new optimizer cost factors.

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

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• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 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)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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Do your changes match any of the following descriptions?

• Delete files
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[gemini-code-assist]

Hello @terry1purcell, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

This pull request adds documentation for new optimizer cost factors in TiDB v9.0.0. These factors are used internally in the Cost Model and are generally not recommended for modification unless directed by TiDB Optimizer Engineering. The changes involve adding descriptions, scope, applicability to hints, data type, range, and default values for each new cost factor.

Highlights

• Documentation: Adds documentation for new optimizer cost factors in system-variables.md.
• New Cost Factors: Introduces documentation for tidb_opt_hash_agg_cost_factor, tidb_opt_hash_join_cost_factor, tidb_opt_index_join_cost_factor, tidb_opt_index_lookup_cost_factor, tidb_opt_index_merge_cost_factor, tidb_opt_index_reader_cost_factor, tidb_opt_index_scan_cost_factor, tidb_opt_limit_cost_factor, tidb_opt_merge_join_cost_factor, tidb_opt_sort_cost_factor, tidb_opt_stream_agg_cost_factor, tidb_opt_table_fulL_scan_cost_factor, tidb_opt_table_range_scan_cost_factor, tidb_opt_table_reader_cost_factor, tidb_opt_table_rowid_cost_factor, tidb_opt_table_tiflash_scan_cost_factor, and tidb_opt_topn_cost_factor.
• Version: Specifies that these cost factors are new in TiDB v9.0.0.

Changelog

Click here to see the changelog

• system-variables.md
  • Added documentation for tidb_opt_hash_agg_cost_factor.
  • Added documentation for tidb_opt_hash_join_cost_factor.
  • Added documentation for tidb_opt_index_join_cost_factor.
  • Added documentation for tidb_opt_index_lookup_cost_factor.
  • Added documentation for tidb_opt_index_merge_cost_factor.
  • Added documentation for tidb_opt_index_reader_cost_factor.
  • Added documentation for tidb_opt_index_scan_cost_factor.
  • Added documentation for tidb_opt_limit_cost_factor.
  • Added documentation for tidb_opt_merge_join_cost_factor.
  • Added documentation for tidb_opt_sort_cost_factor.
  • Added documentation for tidb_opt_stream_agg_cost_factor.
  • Added documentation for tidb_opt_table_fulL_scan_cost_factor.
  • Added documentation for tidb_opt_table_range_scan_cost_factor.
  • Added documentation for tidb_opt_table_reader_cost_factor.
  • Added documentation for tidb_opt_table_rowid_cost_factor.
  • Added documentation for tidb_opt_table_tiflash_scan_cost_factor.
  • Added documentation for tidb_opt_topn_cost_factor.

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---

Cost factors defined,
Optimizer's choices refined,
Queries now take flight.

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[gemini-code-assist]

Code Review

The pull request adds documentation for new optimizer cost factors in TiDB v9.0.0. The changes seem straightforward and well-organized.

Summary of Findings

• Typographical error: There's a typographical error in one of the variable names: tidb_opt_table_fulL_scan_cost_factor. It should be tidb_opt_table_full_scan_cost_factor.
• Inconsistent capitalization: The capitalization of 'fulL' in tidb_opt_table_fulL_scan_cost_factor is inconsistent with other variable names. It should be full.
• Missing period at the end of the sentence: The description for each variable lacks a period at the end of the sentence.

Merge Readiness

The pull request is almost ready for merging. However, the typographical error in the variable name tidb_opt_table_fulL_scan_cost_factor needs to be corrected. Additionally, adding a period at the end of each variable description would improve the documentation's clarity. I am unable to approve this pull request, and recommend that another reviewer approves this code before merging.

[ti-chi-bot]

@gemini-code-assist[bot]: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Code Review

The pull request adds documentation for new optimizer cost factors in TiDB v9.0.0. The changes seem straightforward and well-organized.

Summary of Findings

• Typographical error: There's a typographical error in one of the variable names: tidb_opt_table_fulL_scan_cost_factor. It should be tidb_opt_table_full_scan_cost_factor.
• Inconsistent capitalization: The capitalization of 'fulL' in tidb_opt_table_fulL_scan_cost_factor is inconsistent with other variable names. It should be full.
• Missing period at the end of the sentence: The description for each variable lacks a period at the end of the sentence.

Merge Readiness

The pull request is almost ready for merging. However, the typographical error in the variable name tidb_opt_table_fulL_scan_cost_factor needs to be corrected. Additionally, adding a period at the end of each variable description would improve the documentation's clarity. I am unable to approve this pull request, and recommend that another reviewer approves this code before merging.

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.

[ti-chi-bot]

@qw4990: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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.

[terry1purcell]

/cc @breezewish

[Oreoxmt]

@terry1purcell PTAL terry1purcell#1 and help confirm whether these system variables apply to TiDB Cloud. If they don't, we should to add the following note to each of them:

docs/system-variables.md

Lines 4691 to 4693 in 689ea88

	> **Note:**
	>
	> This TiDB variable is not applicable to TiDB Cloud.

[terry1purcell]

@terry1purcell PTAL terry1purcell#1 and help confirm whether these system variables apply to TiDB Cloud. If they don't, we should to add the following note to each of them:

docs/system-variables.md

Lines 4691 to 4693 in 689ea88

	> **Note:**
	>
	> This TiDB variable is not applicable to TiDB Cloud.

@Oreoxmt - there is nothing specific about these that would prohibit them from being utilized by TiDB cloud - unless we somehow place restrictions on what variables we expose to users. Do you know how I can validate this?

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-07-08 09:42:28.782448131 +0000 UTC m=+1992801.505627110: ☑️ agreed by dveeden.
• 2025-07-16 03:33:15.076401506 +0000 UTC m=+2661847.799580487: ☑️ agreed by Oreoxmt.

[Oreoxmt]

/cc @lilin90