variable: mark tidb_index_serial_scan_concurrency as deprecated

[0xPoe]

What problem does this PR solve?

Issue Number: close #63931

Problem Summary:

What changed and how does it work?

Mark tidb_index_serial_scan_concurrency as deprecated.

We decided in #16999 to deprecate tidb_index_serial_scan_concurrency for controlling the concurrency of sequential scans, but retain its function of controlling the concurrency of index statistics scans.

#50639 We decided to stop using tidb_index_serial_scan_concurrency and instead use the tidb_analyze_distsql_scan_concurrency parameter newly added in the previous PR to control the scan concurrency for index statistical information collection.

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.

将已经没作用的 tidb_index_serial_scan_concurrency 标记为弃用
Mark the obsolete tidb_index_serial_scan_concurrency variable as deprecated

[0xPoe]

/retest

[Copilot]

Pull Request Overview

Mark tidb_index_serial_scan_concurrency as deprecated and ensure it no longer affects execution while still being settable (with a warning).

• Make tidb_index_serial_scan_concurrency a no-op at session level and emit a deprecation warning on SET.
• Remove internal concurrency field/methods tied to index serial scan concurrency.
• Update tests and comments to reflect deprecation and new guidance.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/sessionctx/variable/sysvar.go	Deprecates tidb_index_serial_scan_concurrency by making SetSession a no-op and adding a validation warning.
pkg/sessionctx/variable/session.go	Removes indexSerialScanConcurrency field and its getters/setters from Concurrency.
pkg/sessionctx/variable/varsutil_test.go	Removes tests asserting behavior of the deprecated variable.
pkg/executor/set_test.go	Adjusts tests to expect the deprecation warning and drops assertions on internal concurrency effect.
pkg/sessionctx/vardef/tidb_vars.go	Adds deprecation and historical behavior documentation for the variable and related analyze scan variable.

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.8291%. Comparing base (31338c0) to head (c2bb03e).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #64023        +/-   ##
================================================
+ Coverage   72.7400%   73.8291%   +1.0891%     
================================================
  Files          1851       1854         +3     
  Lines        500444     505032      +4588     
================================================
+ Hits         364023     372861      +8838     
+ Misses       114279     110265      -4014     
+ Partials      22142      21906       -236     

Flag	Coverage Δ
integration	42.3978% <100.0000%> (?)
unit	72.8925% <100.0000%> (+0.6090%)	⬆️

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

Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	46.4659% <ø> (+0.0861%)	⬆️

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[0xPoe]

🔢 Self-check (PR reviewed by myself and ready for feedback)

• Code compiles successfully

• Unit tests added

• All tests pass

• Bazel files updated

• Comments added where necessary

• PR title and description updated

• Documentation PR created (or confirmed not needed)

• PR size is reasonable

/cc @elsa0520 @time-and-fate

[elsa0520]

LGTM

[0xPoe]

/retest

[0xPoe]

/hold

I'd like to update more comments for more variables.

[0xPoe]

/unhold

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-20 08:46:53.344498946 +0000 UTC m=+688719.421751507: ☑️ agreed by elsa0520.
• 2025-10-20 10:34:33.85561573 +0000 UTC m=+695179.932868289: ☑️ agreed by time-and-fate.

[0xPoe]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elsa0520, time-and-fate, yudongusa

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Needs approval from an approver in each of these files:

• OWNERS [elsa0520,time-and-fate]
• pkg/sessionctx/vardef/OWNERS [yudongusa]
• pkg/sessionctx/variable/OWNERS [yudongusa]

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[lance6716]

/retest

[0xPoe]

/test all

[0xPoe]

/retest

[0xPoe]

/test all

[tiprow]

@0xPoe: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
fast_test_tiprow	c2bb03e	link	true	/test fast_test_tiprow

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