ddl: optimize temp index worker in highly conflicting case (#61445)

[ti-chi-bot]

This is an automated cherry-pick of #61445

What problem does this PR solve?

Issue Number: close #61433

Problem Summary:

When there are lots of conflicts between DDL internal txn and DML txn, temp index worker can hardly make any progress.

What changed and how does it work?

• Remove the LockKeys in DDL internal txn from merge index worker.
  • if both DDL txn and DML txn modify the same index key, there will be a conflict and DDL txn will retry. (after #62387)
  • if DDL txn modifies the index key, DML txn only modifies the row key, there will be no conflict anymore (this is the key improvement).
• Delete temp index records after applying them to the original index to eliminate reentrant cost.
• Reduce the batch count of DDL txn when suffering frequent conflicts.
• Add metrics to monitor the progress of merging temp index:
  • tidb_ddl_temp_index_write: the counter of writing to temp index (single write).
  • tidb_ddl_temp_index_double_write: the counter of writing to temp index (double write).
  • tidb_ddl_temp_index_scan: the counter of scanned temp index value records.
  • tidb_ddl_temp_index_merge: the counter of merged temp index value records.

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

[ti-chi-bot]

@tangenta This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[Benjamin2037]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Benjamin2037, fzzf678

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Details

Needs approval from an approver in each of these files:

• OWNERS [Benjamin2037]
• pkg/ddl/OWNERS [Benjamin2037]
• pkg/infoschema/OWNERS [Benjamin2037]
• pkg/table/OWNERS [Benjamin2037]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-07-18 04:18:50.041871527 +0000 UTC m=+2837382.765050512: ☑️ agreed by Benjamin2037.
• 2025-07-18 04:21:48.630058555 +0000 UTC m=+2837561.353237536: ☑️ agreed by fzzf678.

[codecov]

Codecov Report

❌ Patch coverage is 70.77465% with 83 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-8.5@43592e8). Learn more about missing BASE report.

Additional details and impacted files

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

Components	Coverage Δ
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parser	∅ <0.0000%> (?)
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[Benjamin2037]

/retest

[Benjamin2037]

/unhold

[Benjamin2037]

/retest

[fzzf678]

/retest

[hawkingrei]

/retest

[Benjamin2037]

/retest