bvolpato opened a new issue, #26444:
URL: https://github.com/apache/datafusion/issues/26444
### Describe the bug
With TopK aggregation enabled, repeated identical NaN grouping keys can
produce separate groups. Evicting a NaN group can also panic with `internal
error: entered unreachable code` in `aggregates/topk/hash_table.rs`.
### To Reproduce
Reproduced on main at `5f2158269e24ac45e2740dcfe6e79a6f69f28330`:
```sql
SET datafusion.execution.target_partitions = 1;
SET datafusion.optimizer.enable_topk_aggregation = true;
SELECT k, MAX(v) FROM (VALUES
(CAST('NaN' AS DOUBLE), 1),
(CAST('NaN' AS DOUBLE), 2),
(CAST(0 AS DOUBLE), 3)
) AS t(k, v)
GROUP BY k ORDER BY MAX(v) DESC NULLS LAST LIMIT 3;
```
This returns an extra group:
```text
0 3
NaN 2
NaN 1
```
Changing `LIMIT 3` to `LIMIT 1` triggers the eviction panic. Disabling
`datafusion.optimizer.enable_topk_aggregation` gives the expected results.
### Expected behavior
The query with `LIMIT 3` returns `0, 3` and `NaN, 2`. With `LIMIT 1`, it
returns only `0, 3`, without a panic.
### Additional context
#16256 fixed analogous NaN key equality in ordinary aggregation. This report
concerns TopK's key lookup and eviction paths. It uses repeated NaNs with
identical bits and does not require the broader all-NaN canonicalization
proposed in #26091.
### Willingness to contribute
I plan to contribute a fix for this bug independently. A draft PR with the
fix and regression coverage will follow.
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