domain: change @@tidb_schema_cache_size to 0 should not set the capacity to 0

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #59949

Problem Summary:

Change schema cache size take effect immediately, but switch from v2 to v1 take effect after a schema reload.
This cause a bug that during the period when @@tidb_schema_cache_size is set to 0, but the implement have not change to v1, the schame cache hit rate become 0.

What changed and how does it work?

When set @@tidb_schema_cache_size to 0, do not change the schema cache capacity.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Now the result is expected, there is no data after changing from v2 to v1:

• 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

[tiprow]

Hi @tiancaiamao. Thanks for your PR.

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[GMHDBJD]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD, wjhuang2016

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-03-07 07:14:24.696590226 +0000 UTC m=+598577.825509962: ☑️ agreed by wjhuang2016.
• 2025-03-07 07:18:16.014955046 +0000 UTC m=+598809.143874792: ☑️ agreed by GMHDBJD.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.0379%. Comparing base (e6b4d95) to head (5069836).
Report is 15 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59952        +/-   ##
================================================
+ Coverage   72.9252%   75.0379%   +2.1126%     
================================================
  Files          1701       1749        +48     
  Lines        469722     480845     +11123     
================================================
+ Hits         342546     360816     +18270     
+ Misses       106059      97407      -8652     
- Partials      21117      22622      +1505     

Flag	Coverage Δ
integration	49.2976% <100.0000%> (?)
unit	72.3640% <100.0000%> (+0.2275%)	⬆️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	64.3023% <ø> (+19.4718%)	⬆️

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[tiancaiamao]

/retest

[tiprow]

@tiancaiamao: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[tiancaiamao]

/retest

[tiprow]

@tiancaiamao: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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[hawkingrei]

/ok-to-test

[hawkingrei]

/retest