planner: use addedRows for out of range and add skew risk ratio | tidb-test=pr/2646

[ti-chi-bot]

This is an automated cherry-pick of #62363

What problem does this PR solve?

Issue Number: ref #62093 close #55199

Problem Summary:

What changed and how does it work?

Range predicates outside of the histogram bucket range face a similar risk to ranges within a bucket, because we do not have collected stats we can rely on for the cardinality estimation. So, this change uses the tidb_opt_risk_range_skew_ratio variable to allow users or external processes to adjust how much skew is considered in out of range estimations. This also adjusts and simplifies the out of range estimation function to rely more on addedRows (the difference between realTimeRowCount and the histogram count) than histogram row count itself.

Previously, when estimating a predicate outside of our histogram bucket range, we would calculate a percentage, TotalPercent, and multiply it with histogram row count. In the case that addedRows was significant, we would multiply TotalPercent and addedRows and take the larger of the two estimations. Finally, we would ensure our estimate didn't go over ModifyCount. Part of the concern with using histogram row count is that one-sided ranges, such as a > 0, could easily have a TotalPercent = 50%, leading to large out of range estimations. It seems more reasonable for out of range cardinality to be correlated with addedRows rather than histogram row count, no matter the scenario. So, to simplify the logic, the estimation is changed to be TotalPercent * addedRows * 0.5 by default (the 0.5 accounts for the assumption that only half of newly added rows are added outside our histogram range).

To account for underestimation due to skew, since it is possible that all of our newly added rows are within our queried range, we can use the tidb_opt_risk_range_skew_ratio variable. Based on the value of this variable, we account for more or less skew in our estimate. For a default value of 0, there is no change to the original estimate, and for a value of 1, our out of range estimation = worst case skew = addedRows.

Check List

Tests

• [x] 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

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: fixdb Once this PR has been reviewed and has the lgtm label, please assign yudongusa for approval. For more information see the Code Review Process. Please ensure that each of them provides their approval before proceeding.

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Needs approval from an approver in each of these files:

• ~~OWNERS~~ [fixdb]
• ~~pkg/planner/OWNERS~~ [fixdb]
• pkg/sessionctx/variable/OWNERS
• ~~pkg/statistics/OWNERS~~ [fixdb]

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-12-09 02:38:16.727574868 +0000 UTC m=+922241.541352430: :ballot_box_with_check: agreed by fixdb.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review. :warning: Please upload report for BASE (release-8.5@07dd0cb). Learn more about missing BASE report.

Additional details and impacted files

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##             release-8.5     #64784   +/-   ##
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Flag	Coverage Δ
integration	49.7175% <68.9655%> (?)

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Components	Coverage Δ
dumpling	∅ <0.0000%> (?)
parser	∅ <0.0000%> (?)
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[fixdb]

/retest

[terry1purcell]

/unhold

[hawkingrei]

/retest

[ti-chi-bot[bot]]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	aa60835d0839d27840a6be216b765f0e03f3678b	link	true	/test unit-test
idc-jenkins-ci-tidb/mysql-test	aa60835d0839d27840a6be216b765f0e03f3678b	link	true	/test mysql-test

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[ti-chi-bot[bot]]

This cherry pick PR is for a release branch and has not yet been approved by triage owners. Adding the do-not-merge/cherry-pick-not-approved label.

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[tiprow[bot]]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	aa60835d0839d27840a6be216b765f0e03f3678b	link	true	/test fast_test_tiprow_for_release

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