ddl: fix a bug about setting MaxIndexLength

[zimulala]

What problem does this PR solve?

Issue Number: close #53281

Problem Summary:
Both scenarios are explained in the issue.

Introduce code for this issue: https://github.com/pingcap/tidb/pull/50624/files#diff-45ad039a8b87835b475a52615355c444538d3a1f31a84d7b3ac94e15b1ce8721R126-R127

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

[tiprow]

Hi @zimulala. Thanks for your PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.9529%. Comparing base (6aef624) to head (8b1dce0).
Report is 7 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #53284        +/-   ##
================================================
+ Coverage   72.4435%   74.9529%   +2.5093%     
================================================
  Files          1493       1497         +4     
  Lines        429362     432661      +3299     
================================================
+ Hits         311045     324292     +13247     
+ Misses        99037      88435     -10602     
- Partials      19280      19934       +654     

Flag	Coverage Δ
integration	49.8580% <100.0000%> (?)
unit	71.7219% <100.0000%> (+0.3875%)	⬆️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	50.4818% <ø> (+9.0483%)	⬆️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, tangenta

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• pkg/ddl/OWNERS [tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-15 04:26:21.364238863 +0000 UTC m=+1627335.121374435: ☑️ agreed by Defined2014.
• 2024-05-15 06:08:18.337950202 +0000 UTC m=+1633452.095085776: ☑️ agreed by tangenta.

[zimulala]

/retest-required

[tiprow]

@zimulala: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest-required

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[bb7133]

/run-cherry-picker

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #53408.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #53409.