planner,ddl: rightly fix issue of add foreign key too slow in big table

[hawkingrei]

What problem does this PR solve?

Issue Number: ref #40111

Problem Summary:

What changed and how does it work?

Old bugfix has problem. The root cause is the uninit system variable.

Check List

Tests

• Unit test

in the origin Bugfix PR，TestAddForeignKeyInBigTable is for testing this problem.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 75.5456%. Comparing base (1b35bd0) to head (fc11e9a).
Report is 15 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61945        +/-   ##
================================================
+ Coverage   72.9655%   75.5456%   +2.5800%     
================================================
  Files          1735       1781        +46     
  Lines        481848     495682     +13834     
================================================
+ Hits         351583     374466     +22883     
+ Misses       108725      98349     -10376     
- Partials      21540      22867      +1327     

Flag	Coverage Δ
integration	49.3520% <100.0000%> (?)
unit	72.8230% <100.0000%> (+0.6141%)	⬆️

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

Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	64.2186% <ø> (+17.6266%)	⬆️

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-06-24 02:30:38.293585405 +0000 UTC m=+757291.016764382: ☑️ agreed by crazycs520.
• 2025-06-24 06:15:01.057599836 +0000 UTC m=+770753.780778813: ☑️ agreed by tangenta.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: crazycs520, tangenta, yudongusa

The full list of commands accepted by this bot can be found here.

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Details

Needs approval from an approver in each of these files:

• pkg/ddl/OWNERS [tangenta]
• pkg/sessionctx/variable/OWNERS [yudongusa]

Approvers can indicate their approval by writing /approve in a comment
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[hawkingrei]

/retest