[bvolpato]

Which issue does this PR close?

Closes #26444.

Rationale for this change

Repeated identical NaN keys can form separate TopK groups, and evicting a NaN group can panic. Valid GROUP BY ... ORDER BY ... LIMIT queries can therefore return incorrect results or fail.

What changes are included in this PR?

• Compare primitive grouping keys with Arrow equality so identical NaN payloads match.
• Normalize signed zero consistently with ordinary grouping while preserving distinct NaN payloads.
• Remove occupied entries by their known store index during eviction and NULL aggregate transitions.

A separate prerequisite commit retains the RSS sampling fix from #26208. It samples the current process before query execution so short queries cannot leave the recorded maximum at zero. Memory thresholds are unchanged.

What is the testing strategy for this PR?

The SQL regressions cover repeated NaN keys and eviction. Applied to current main (7f493a565), the eviction regression fails with internal error: entered unreachable code; it passes with this fix. Hash-table tests cover Float16/32/64, distinct NaN payloads, signed zero, and NULL aggregate transitions.

The extended workspace suite, full SQL logic suite, and cargo test --profile ci -p datafusion-cli passed. cargo fmt --all, cargo clippy --all-targets --all-features -- -D warnings, and uv run ./dev/rust_lint.sh also passed.

The existing topk_aggregate primitive-key case showed no significant change against main. This comparison used the unoptimized CI profile on a shared host, so its performance evidence is limited.

Are there any user-facing changes?

Identical NaN keys form one TopK group, and evicting a NaN group succeeds. Signed-zero grouping matches ordinary aggregation. There are no public API changes.

AI-assisted contribution.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.54930% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.93%. Comparing base (7f493a5) to head (fdd9fd1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/physical-plan/src/aggregates/topk/hash_table.rs	91.54%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26445   +/-   ##
=======================================
  Coverage   82.93%   82.93%           
=======================================
  Files        1154     1154           
  Lines      457851   457915   +64     
  Branches   457851   457915   +64     
=======================================
+ Hits       379701   379761   +60     
  Misses      55131    55131           
- Partials    23019    23023    +4     

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