distsql: do not change concurrency for keep order request when @@tidb_distsql_scan_concurrency is set

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #60891

Problem Summary:

What changed and how does it work?

In #35975, distsql scan concurrency is set to 2 for keep order query.
And later, the PR is partly reset in #55196, concurrency is change to 2 only in limited cases.

It should work most of the times, but in some corner cases the user want better performance and do not care much about the meomry usage, we should have an option for them.
In this PR, if the value of @@tidb_distsql_scan_concurrency is revised, obey that setting.
So when user found a case, he case workaround using hint for that query:

select /*+ set_var(tidb_distsql_scan_concurrency=14)*/ * from t order by id;

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.0644%. Comparing base (b81a9df) to head (98195dd).
Report is 460 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #60803        +/-   ##
================================================
+ Coverage   73.1201%   76.0644%   +2.9442%     
================================================
  Files          1716       1724         +8     
  Lines        475725     505457     +29732     
================================================
+ Hits         347851     384473     +36622     
+ Misses       106490      99807      -6683     
+ Partials      21384      21177       -207     

Flag	Coverage Δ
integration	45.2153% <100.0000%> (?)
unit	75.0655% <100.0000%> (+2.7246%)	⬆️

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Components	Coverage Δ
dumpling	53.0684% <ø> (+0.4130%)	⬆️
parser	∅ <ø> (∅)
br	50.1161% <ø> (+2.7370%)	⬆️

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[tiancaiamao]

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[tiancaiamao]

/test unit-test

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[tiancaiamao]

/check-issue-triage-complete

[cfzjywxk]

Should the document be upated too?

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, XuHuaiyu

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[LGTM Timeline notifier]

Timeline:

• 2025-05-15 06:40:14.792786589 +0000 UTC m=+8381.048683985: ☑️ agreed by XuHuaiyu.
• 2025-05-16 06:33:48.467874041 +0000 UTC m=+94394.723771438: ☑️ agreed by cfzjywxk.

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #61150.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #62276.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #62280.