ddl: Exchange part schema load fix (#46126)

[ti-chi-bot]

This is an automated cherry-pick of #46126

What problem does this PR solve?

Issue Number: ref #46125 close #45791

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Manual test:
Build this PR and use it for starting a cluster tiup playground --db.binpath ./bin/tidb-server
and then run tiflash test sh run-test.sh fullstack-test2/ddl/alter_exchange_partition.test which no longer fails
Also check the tidb.log that there are no failures loading new schema diffs, so no full schema reloads are needed.

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

(Unless 7.4 get released without this fix).

[mjonss]

/retest

[mjonss]

/retest

[JaySon-Huang]

/reopen

[ti-chi-bot]

@JaySon-Huang: Reopened this PR.

In response to this:

/reopen

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[JaySon-Huang]

Reopen it because #45980 is backport to release-6.5 and it make TiFlash hangs after executing exchange partition

[JaySon-Huang]

/test unit-test

[tiprow]

@JaySon-Huang: No presubmit jobs available for pingcap/tidb@release-6.5

In response to this:

/test unit-test

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[JaySon-Huang]

/retest

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@ec0ffe8). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-6.5     #46191   +/-   ##
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  Coverage               ?   73.6675%           
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  Files                  ?       1084           
  Lines                  ?     348304           
  Branches               ?          0           
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  Hits                   ?     256587           
  Misses                 ?      75247           
  Partials               ?      16470           

[bb7133]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, mjonss

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-15 11:31:18.391933996 +0000 UTC m=+257844.359522046: ☑️ agreed by mjonss.
• 2023-09-16 15:26:33.316449348 +0000 UTC m=+358359.284037398: ☑️ agreed by bb7133.

[bb7133]

/merge

[ti-chi-bot]

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