planner: Add tidb_max_estimated_cost to prevent trying to run too expensive queries

[mjonss]

What problem does this PR solve?

Issue Number: close #64052

Problem Summary:

What changed and how does it work?

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.

Added `tidb_max_estimated_cost` to not execute queries with too high costs.

[ti-chi-bot]

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[tiprow]

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[Copilot]

Pull Request Overview

This PR adds a new system variable tidb_max_estimated_cost to prevent execution of queries whose optimizer-estimated cost exceeds a specified threshold. This provides a mechanism to protect the database from running potentially expensive queries that could impact performance.

Key changes:

• Introduces tidb_max_estimated_cost as a session/global variable with default value 0.0 (disabled)
• Implements cost validation in both CascadesOptimize and physicalOptimize functions
• Adds comprehensive tests for the variable configuration and query execution behavior

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
pkg/sessionctx/vardef/tidb_vars.go	Defines the new TiDBMaxEstimatedCost variable constant and its default value
pkg/sessionctx/variable/session.go	Adds MaxEstimatedCost field to SessionVars to track the threshold
pkg/sessionctx/variable/sysvar.go	Registers the system variable with appropriate scope, type, and validation
pkg/sessionctx/variable/sysvar_test.go	Tests variable validation, setting session/global values, and error handling
pkg/planner/core/optimizer.go	Implements cost checking logic in both optimization paths, rejecting queries that exceed the threshold
pkg/planner/core/plan_cost_ver1_test.go	Integration tests verifying cost-based query rejection with different cost models and scenarios

[codecov]

Codecov Report

❌ Patch coverage is 51.85185% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.0514%. Comparing base (cea660d) to head (3c43b3f).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #64580        +/-   ##
================================================
- Coverage   74.6960%   73.0514%   -1.6447%     
================================================
  Files          1888       1866        -22     
  Lines        514956     506984      -7972     
================================================
- Hits         384652     370359     -14293     
- Misses       106483     114314      +7831     
+ Partials      23821      22311      -1510     

Flag	Coverage Δ
integration	41.7349% <51.8518%> (-6.4205%)	⬇️

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	46.0770% <ø> (-17.1269%)	⬇️

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[mjonss]

/retest

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[ti-chi-bot]

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

Test name	Commit	Details	Required	Rerun command
pull-unit-test-next-gen	3c43b3f	link	true	/test pull-unit-test-next-gen
idc-jenkins-ci-tidb/unit-test	3c43b3f	link	true	/test unit-test

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