topsql: move fast plan check to beginning of observeStmtBeginForTopSQL when disable top sql

[VGalaxies]

What problem does this PR solve?

Issue Number: close #60604

Problem Summary: see #60604

What changed and how does it work?

The fast plan check is now performed at the very beginning of observeStmtBeginForTopSQL. This change skips the expensive GetPlanDigest call when TopSQL is disabled and the execution plan is a fast path, since point gets return early and the plan digest is only cached without further use.

For this optimization to take effect, TopSQL, slow log, and statement summary must be disabled.

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.

before:

after (no GetPlanDigest found in flame graph):

in our special edition of TiDB, sysbench point select QPS can have 2~3% increment

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

[sre-bot]

All committers have signed the CLA.

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Comments suppressed due to low confidence (1)

pkg/executor/adapter.go:2162

• Ensure that the early return when a fast plan is detected does not bypass any essential logging or metric updates that might be needed for diagnostic purposes in production.

if (!topsqlstate.TopSQLEnabled() && IsFastPlan(a.Plan)) {

[lance6716]

/ok-to-test

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: crazycs520, lance6716

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-04-16 09:33:23.677195707 +0000 UTC m=+2853697.361431803: ☑️ agreed by lance6716.
• 2025-04-16 09:38:22.726907491 +0000 UTC m=+2853996.411143587: ☑️ agreed by crazycs520.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.4986%. Comparing base (4f2d944) to head (8146832).
Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #60605        +/-   ##
================================================
+ Coverage   73.0969%   73.4986%   +0.4017%     
================================================
  Files          1715       1715                
  Lines        475094     475135        +41     
================================================
+ Hits         347279     349218      +1939     
+ Misses       106453     104552      -1901     
- Partials      21362      21365         +3     

Flag	Coverage Δ
integration	42.8213% <100.0000%> (?)
unit	72.3140% <100.0000%> (-0.0139%)	⬇️

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

Components	Coverage Δ
dumpling	52.6553% <ø> (ø)
parser	∅ <ø> (∅)
br	47.2688% <ø> (+0.0304%)	⬆️

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #60607.