Planner: Do not allow cardinality to go below 1 | tidb-test=pr/2450

[ti-chi-bot]

This is an automated cherry-pick of #55242

What problem does this PR solve?

Issue Number: close #47400

Problem Summary:

What changed and how does it work?

Ensure that the cardinality estimate does not go below 1. As the estimate approaches zero, then all plans are inexpensive which makes it difficult for the optimizer to differentiate the plan choices.

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

[terry1purcell]

/retest

[terry1purcell]

/retest

[hawkingrei]

/retest

[Rustin170506]

Thanks!

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, Rustin170506

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Needs approval from an approver in each of these files:

• OWNERS [Rustin170506,hawkingrei]
• pkg/planner/OWNERS [Rustin170506,hawkingrei]
• pkg/statistics/OWNERS [Rustin170506,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-12 09:41:29.420852605 +0000 UTC m=+517879.509655145: ☑️ agreed by hawkingrei.
• 2025-02-21 09:32:10.197675075 +0000 UTC m=+2678.150833340: ☑️ agreed by Rustin170506.

[Rustin170506]

/retest

[Rustin170506]

/retest

[Rustin170506]

/retest

[Rustin170506]

The unit test failed:

        	            	Diff:

        	            	--- Expected

        	            	+++ Actual

        	            	@@ -1,3 +1,3 @@

        	            	-[IndexReader_6 0.00 root  index:IndexRangeScan_5]

        	            	-[└─IndexRangeScan_5 0.00 cop[tikv] table:exp_backoff, index:idx(a, b, c, d) range:[1 1 1 3,1 1 1 5], keep order:false]

        	            	+[IndexReader_6 1.00 root  index:IndexRangeScan_5]

        	            	+[└─IndexRangeScan_5 1.00 cop[tikv] table:exp_backoff, index:idx(a, b, c, d) range:[1 1 1 3,1 1 1 5], keep order:false]

[hawkingrei]

/retest

[ti-chi-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/mysql-test	a7009cd	link	true	/test mysql-test
idc-jenkins-ci-tidb/unit-test	a7009cd	link	true	/test unit-test

Full PR test history. Your PR dashboard.

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[hawkingrei]

/hold

[ti-chi-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.

To merge this cherry pick:

1. It must be approved by the approvers firstly.
2. AFTER it has been approved by approvers, please wait for the cherry-pick merging approval from triage owners.

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