fix: prevent panic in valueDec128Compare when comparing decimal with integer types

[jiangxinmeng1]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #24081

What this PR does / why we need it:

When executing a window function query like:

SELECT ... FROM (
    SELECT ROW_NUMBER() OVER (...) AS rn,
           SUM(amount) OVER (...) AS cum_val  -- amount is DECIMAL(10,2)
    FROM orders
) AS x
WHERE x.rn = 2;

The filter x.rn = 2 panics with:

panic unsafe slice cast: from length 8 is not enough for length 16

Root cause: The comparison dispatcher (equalFn) selects valueDec128Compare based on one parameter's type (decimal128). When the other parameter is int64 (from ROW_NUMBER()), it tries to read 8-byte int64 data as 16-byte Decimal128, causing a panic.

Fix: Extend the existing type-mismatch fallback mechanism in func_compare_fix.go:

• Add T_decimal64 and T_decimal128 to isNumericType() so shouldUseTypeMismatchPath() catches decimal-vs-integer mismatches
• Add decimal conversion cases in getAsFloat64Slice() using Decimal64ToFloat64 / Decimal128ToFloat64

Test coverage

Added tests:

• TestEqualFnDecimalIntMismatch — reproduces the exact panic scenario (decimal128 == int64, int64 == decimal128, decimal64 == decimal128)
• Updated TestIsNumericType and TestShouldUseTypeMismatchPath for decimal types
• Added decimal→float64 conversion tests in TestGetAsFloat64Slice

[XuPeng-SH]

Code Review Summary

LGTM ✅

Changes Reviewed

1. isNumericType() - Correctly extended to include T_decimal64 and T_decimal128
2. getAsFloat64Slice() - Properly added decimal→float64 conversion using Decimal64ToFloat64() and Decimal128ToFloat64() with correct Scale handling
3. Test coverage - Comprehensive tests for all new functionality

Root Cause

When comparing decimal128 with int64 (e.g., ROW_NUMBER() = 2), the comparison function tried to read 8-byte int64 data as 16-byte Decimal128, causing panic:

panic unsafe slice cast: from length 8 is not enough for length 16

Fix Mechanism

The fix correctly leverages the existing type-mismatch fallback path by:

1. Recognizing decimal types as numeric in isNumericType()
2. Converting decimal values to float64 for comparison when type mismatch is detected

Potential Note

Decimal→float64 conversion has precision loss (38 digits → 15-16 digits), but this is acceptable for the fallback path. For ROW_NUMBER() comparisons (integer values), precision is preserved.

All tests pass ✅

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-09 03:10 UTC · Rule: release-3.0
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-04-09 03:10 UTC · at 9e20b7a5712c50c8f7833b08c3f8f0b0f26bff75

This pull request spent 13 seconds in the queue, including 1 second running CI.

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