planner: reduce within bucket usage of range risk

[terry1purcell]

What problem does this PR solve?

Issue Number: ref #65294

Problem Summary:

What changed and how does it work?

Variable tidb_opt_risk_range_skew_ratio can be used to adjust the estimation in 2 scenarios:

• out of range estimation
• within a histogram bucket estimate
  Customers attempting to use this variable to adjust only the out-of-range estimate have incurred unexpected plan changes due to the impact on "within bucket estimation".
  Given that out-of-range is the more critical customer issue - this PR reduces the impact that tidb_opt_risk_range_skew_ratio has on within bucket estimation, and adds a TODO to note that further work is required if customers also experience estimation issues within histogram buckets.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

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[terry1purcell]

/ok-to-test

[Copilot]

Pull request overview

Adjusts histogram range row-count estimation to reduce the effect of tidb_opt_risk_range_skew_ratio on in-bucket estimates, while still tracking a “worst-case” max estimate for planner risk evaluation.

Changes:

• Switches BetweenRowCount’s internal rangeEst computation to use RowEstimate directly throughout.
• Modifies the in-bucket skew handling to cap how much RiskRangeSkewRatio can inflate the default estimate, and updates MaxEst for risk-based comparisons.
• Adds a TODO note documenting the intent and future follow-up area.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.3149%. Comparing base (e8a60bd) to head (e457c1e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #65856        +/-   ##
================================================
+ Coverage   77.7750%   78.3149%   +0.5398%     
================================================
  Files          2001       1923        -78     
  Lines        545527     533123     -12404     
================================================
- Hits         424284     417515      -6769     
+ Misses       119581     115165      -4416     
+ Partials       1662        443      -1219     

Flag	Coverage Δ
integration	44.2039% <72.2222%> (-3.9609%)	⬇️
unit	76.7126% <100.0000%> (+0.3013%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	56.7974% <ø> (ø)
parser	∅ <ø> (∅)
br	48.8517% <ø> (-12.1388%)	⬇️

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 4 comments.

[terry1purcell]

/retest-required

[fixdb]

+1

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-28 09:38:18.04733577 +0000 UTC m=+1177925.661292616: ☑️ agreed by fixdb.

[terry1purcell]

/retest-required

[terry1purcell]

/retest-required