planner: out of range common prefix | tidb-test=pr/2630

[terry1purcell]

What problem does this PR solve?

Issue Number: ref #64378

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

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None

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[codecov]

Codecov Report

❌ Patch coverage is 91.89189% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.6265%. Comparing base (e10a603) to head (d17339f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #64510        +/-   ##
================================================
+ Coverage   72.6816%   73.6265%   +0.9448%     
================================================
  Files          1866       1889        +23     
  Lines        506282     514399      +8117     
================================================
+ Hits         367974     378734     +10760     
+ Misses       115918     112741      -3177     
- Partials      22390      22924       +534     

Flag	Coverage Δ
integration	44.4912% <70.2702%> (?)
unit	72.5417% <91.8918%> (+0.2825%)	⬆️

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Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	46.2563% <ø> (+0.0951%)	⬆️

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[terry1purcell]

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[Copilot]

Pull Request Overview

This PR addresses issue #64378 related to "out of range common prefix" estimation in the query planner. The fix improves cardinality estimation when query ranges fall outside histogram bounds, particularly for string/byte comparisons where common prefix comparison results in equal scalar values (l == r).

Key changes:

• Added highIsOpenEnded parameter to OutOfRangeRowCount to detect open-ended ranges (e.g., when upper bound is MaxValue)
• Moved addedRows and modifyCount calculations earlier in the function to enable their use in the l == r case
• Added special handling for l == r scenario with different estimates based on whether the range is open-ended
• Changed the condition from l >= r to separate l > r and l == r cases for more accurate handling

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/statistics/histogram.go	Core logic changes: added highIsOpenEnded parameter, moved calculations, and added special handling for l == r case
pkg/planner/cardinality/row_count_index.go	Updated all call sites to detect and pass highIsOpenEnded flag based on index range high values
pkg/planner/cardinality/row_count_column.go	Updated call site to detect and pass highIsOpenEnded flag based on column range high values
tests/integrationtest/r/planner/cardinality/selectivity.result	Updated expected test results showing increased row count estimates from 1.00 to 2.00 for various queries
tests/integrationtest/r/imdbload.result	Updated expected test results showing increased row count estimates (e.g., from 2.00 to 6.65, from 1.00 to 3.32)

[mjonss]

So this will only change the estimate for open ended queries like WHERE b > 6 but not WHERE b BETWEEN 6 AND 2000000?