planner: fix single mv index merge case couldn't be hinted by use_index_merge (#50572)

[ti-chi-bot]

This is an automated cherry-pick of #50572

What problem does this PR solve?

Issue Number: close #50553

Problem Summary:

What changed and how does it work?

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.6@d98ec8e). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.6     #50595   +/-   ##
================================================
  Coverage               ?   70.0271%           
================================================
  Files                  ?       1436           
  Lines                  ?     419240           
  Branches               ?          0           
================================================
  Hits                   ?     293582           
  Misses                 ?     105365           
  Partials               ?      20293           

Flag	Coverage Δ
unit	70.0271% <0.0000%> (?)

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

Components	Coverage Δ
dumpling	54.0269% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	49.6845% <0.0000%> (?)

[fixdb]

+1

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-19 17:58:55.282743595 +0000 UTC m=+551576.847041298: ☑️ agreed by fixdb.
• 2024-01-21 15:20:32.676133368 +0000 UTC m=+714874.240431073: ☑️ agreed by AilinKid.

[AilinKid]

/test check-dev

[tiprow]

@AilinKid: No presubmit jobs available for pingcap/tidb@release-7.6

In response to this:

/test check-dev

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/test-infra repository.

[AilinKid]

/test check-dev

[tiprow]

@AilinKid: No presubmit jobs available for pingcap/tidb@release-7.6

In response to this:

/test check-dev

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/test-infra repository.

[hawkingrei]

/retest