executor: Fix compact for VARCHAR PK (#51812)

[ti-chi-bot]

This is an automated cherry-pick of #51812

What problem does this PR solve?

Issue Number: Fix #51810

Problem Summary:

What changed and how does it work?

When returning the Start key and End key to TiDB, TiFlash does not encode them in a memory comparable way, so that we cannot simply compare these keys.

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.

Fix `ALTER TABLE ... COMPACT` may not work for tables with a VARCHAR primary key

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.5@68ab772). Click here to learn what that means.
The diff coverage is 0.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #51819   +/-   ##
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  Coverage               ?   71.9856%           
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  Files                  ?       1408           
  Lines                  ?     409090           
  Branches               ?          0           
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  Hits                   ?     294486           
  Misses                 ?      94788           
  Partials               ?      19816           

Flag	Coverage Δ
unit	71.9856% <0.0000%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.3457% <0.0000%> (?)

[windtalker]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: breezewish, windtalker

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-16 02:38:26.09447457 +0000 UTC m=+1085133.116720958: ☑️ agreed by breezewish.
• 2024-04-15 08:52:20.419699497 +0000 UTC m=+241759.530745941: ☑️ agreed by windtalker.