planner: fix get wrong cost with cost tracer (#61196)

[ti-chi-bot]

This is an automated cherry-pick of #61196

What problem does this PR solve?

Issue Number: close #61155

Problem Summary:

When we use the cost trace's explain, it will calculate the cost twice. Unfortunately, when calculating secondly, the row count has been polluted by the first.

the first calculation:

1500000000*math.log2(46.17)*11.6+10000*math.log2(46.17)*11.6 = 96203219945.10272

1500000000 will become the 5.645217633333338e+07, because postOptimize‘s
tryEnableLateMaterialization can change the row count in the first plan generate.

tidb/pkg/planner/core/tiflash_selection_late_materialization.go

Lines 283 to 290 in 7419112

	func PushedDown(sel *PhysicalSelection, ts *PhysicalTableScan, selectedConds []expression.Expression, selectedSelectivity float64) {
	// remove the pushed down conditions from selection
	removeSpecificExprsFromSelection(sel, selectedConds)
	// add the pushed down conditions to table scan
	ts.LateMaterializationFilterCondition = selectedConds
	// Update the row count of table scan after pushing down the conditions.
	ts.StatsInfo().RowCount *= selectedSelectivity
	}

the second calculation:

5.645217633333338e+07*math.log2(46.17)*11.6+10000*math.log2(46.17)*11.6 = 3621204637.5523133

So two formats have different costs.

What changed and how does it work?

Generate the cost trace in the first calculation. Remove the second calculation.

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.

planner: fix get wrong cost with cost tracer

修复使用 cost tracer 得到不正确的 cost 值

[ti-chi-bot]

@hawkingrei This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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Flag	Coverage Δ
integration	37.0281% <100.0000%> (?)
unit	72.7531% <100.0000%> (?)

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Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	52.6880% <0.0000%> (?)

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[hawkingrei]

/retest

[hawkingrei]

/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, hawkingrei

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[AilinKid]

/retest-required

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-07-08 02:33:29.806526389 +0000 UTC m=+1967062.529705366: ☑️ agreed by hawkingrei.
• 2025-07-09 15:11:29.807943839 +0000 UTC m=+2098942.531122820: ☑️ agreed by AilinKid.

[AilinKid]

/retest-required

[AilinKid]

local test can succeed for TestQ22, unit test is ok

[AilinKid]

/retest-required

[AilinKid]

/test unit-test

[tiprow]

@AilinKid: No presubmit jobs available for pingcap/[email protected]

In response to this:

/test unit-test

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[fixdb]

/retest

[hawkingrei]

/retest

[hawkingrei]

/retest

[hawkingrei]

/retest