Do not clobber SQLite's default cost estimates in BestIndex

[mattn]

goVBestIndex unconditionally overwrote estimatedCost and estimatedRows with the IndexResult fields, so implementations that leave them unset reported cost 0 for every candidate plan, breaking query planning. Keep SQLite's pre-initialized defaults unless the implementation set a positive value, and clamp the float-to-int64 conversion of EstimatedRows.

Summary by CodeRabbit

• Bug Fixes
  • Improved query planning by preserving SQLite’s default estimated cost and row counts when custom estimates are not provided.
  • Added safer handling for estimated row counts, including protection against very large values and preventing small fractional estimates from being stored as zero.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 1f0b756 and 2897b83.

📒 Files selected for processing (1)

• sqlite3_opt_vtable.go

🚧 Files skipped from review as they are similar to previous changes (1)

• sqlite3_opt_vtable.go

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📝 Walkthrough

Walkthrough

goVBestIndex preserves SQLite’s default planning estimates unless positive Go-provided values are returned, clamps oversized row counts, and prevents small positive estimates from truncating to zero.

Changes

Virtual table planning

Layer / File(s)	Summary
Conditional estimate handling sqlite3_opt_vtable.go	goVBestIndex conditionally updates estimated cost and rows, clamps row values at math.MaxInt64, and ensures positive fractional rows become at least 1.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main BestIndex change to preserve SQLite’s default cost estimates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit unit tests in branch fix-vtable-estimated-cost

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[coderabbitai]

Actionable comments posted: 1

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Verify each finding against current code. Fix only still-valid issues, skip the
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Inline comments:
In `@sqlite3_opt_vtable.go`:
- Around line 492-499: Update the EstimatedRows handling in the vtable
row-estimate assignment so any positive fractional value below 1 is normalized
to 1 before converting to int64. Preserve the existing MaxInt64 cap and
assignment through info.estimatedRows.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 3babc1f and 1f0b756.

📒 Files selected for processing (1)

• sqlite3_opt_vtable.go