planner: fix leading hint cannot take effect in UNION ALL statements

[hawkingrei]

What problem does this PR solve?

Issue Number: close #50067

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

Merging #50145 (4060b58) into master (9b0fd9e) will decrease coverage by 12.0763%.
Report is 40 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #50145         +/-   ##
=================================================
- Coverage   79.3229%   67.2467%   -12.0763%     
=================================================
  Files          2447       2569        +122     
  Lines        673700     842643     +168943     
=================================================
+ Hits         534399     566650      +32251     
- Misses       117932     252133     +134201     
- Partials      21369      23860       +2491     

Flag	Coverage Δ
integration	36.5399% <100.0000%> (?)
unit	79.2482% <100.0000%> (-0.0748%)	⬇️

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

Components	Coverage Δ
dumpling	73.6130% <ø> (ø)
parser	∅ <ø> (∅)
br	71.7131% <ø> (+3.2370%)	⬆️

[AilinKid]

/test check_dev

[tiprow]

@AilinKid: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test tiprow_fast_test

Use /test all to run all jobs.

In response to this:

/test check_dev

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[elsa0520]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, elsa0520

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Needs approval from an approver in each of these files:

• OWNERS [AilinKid,elsa0520]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-06 09:23:02.50198235 +0000 UTC m=+89572.086236031: ☑️ agreed by AilinKid.
• 2024-01-10 07:15:36.188962973 +0000 UTC m=+427525.773216694: ☑️ agreed by elsa0520.

[elsa0520]

This PR shouldn't change the result of this test

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #50257.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #50258.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #50259.

[ti-chi-bot]

/cherry-pick release-7.6

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #50721.

[ti-chi-bot]

@ti-chi-bot: new pull request could not be created: failed to create pull request against pingcap/tidb#release-7.6 from head ti-chi-bot:cherry-pick-50145-to-release-7.6: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-50145-to-release-7.6."}],"documentation_url":"https://docs.github.com/rest/pulls/pulls#create-a-pull-request"}

In response to this:

/cherry-pick release-7.6

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