planner: increase default tidb_stmt_summary_max_sql_length and better error

[mjonss]

What problem does this PR solve?

Issue Number: close #63671

Problem Summary:
For queries longer than 4096 characters, it will be truncated in the statements_summary tables, resulting in CREATE GLOBAL BINDING FROM HISTORY USING PLAN DIGEST '' does not work, and it gives a cryptic error from the parser.

What changed and how does it work?

Increased the default from 4k to 32k and updated the error message, to point to tidb_stmt_summary_max_sql_length global system variable, controlling the length.

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

  • Only if actually using such long queries.

• 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.

Changed default `tidb_stmt_summary_max_sql_length` from 4096 to 32768 to not truncate medium/large queries in the statements_summary tables.

[tiprow]

Hi @mjonss. Thanks for your PR.

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[mjonss]

/check-issue-triage-complete

[hawkingrei]

/retest

[hawkingrei]

@mjonss Please fix the CI.

[hawkingrei]

/retest

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.9752%. Comparing base (8840daf) to head (a298d25).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #63694        +/-   ##
================================================
+ Coverage   72.7311%   73.9752%   +1.2441%     
================================================
  Files          1848       1849         +1     
  Lines        498587     505327      +6740     
================================================
+ Hits         362628     373817     +11189     
+ Misses       113927     109642      -4285     
+ Partials      22032      21868       -164     

Flag	Coverage Δ
integration	42.9301% <87.5000%> (?)
unit	73.1597% <50.0000%> (+0.8869%)	⬆️

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	46.4741% <ø> (+0.0506%)	⬆️

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[mjonss]

/retest

[tiprow]

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

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[henrybw]

I think this should be fine: I don't think it will add significantly more memory pressure overall. (And if it does turn out to be a problem, we can turn down the default later.)

[henrybw]

I would consider using the vardef.TiDBStmtSummaryMaxSQLLength constant instead of hardcoding tidb_stmt_summary_max_sql_length in the error message, but I don't consider it a blocking issue.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-03 03:03:22.80602704 +0000 UTC m=+399383.062758440: ☑️ agreed by henrybw.
• 2025-10-03 03:21:15.421273161 +0000 UTC m=+400455.678004580: ☑️ agreed by wddevries.

[terry1purcell]

/unhold

[yudongusa]

Please also open a document PR

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: henrybw, terry1purcell, wddevries, yudongusa

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• OWNERS [terry1purcell]
• pkg/planner/OWNERS [henrybw,terry1purcell,wddevries]
• pkg/sessionctx/vardef/OWNERS [yudongusa]

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