gboucher90 opened a new pull request, #20858:
URL: https://github.com/apache/datafusion/pull/20858
Add a deterministic test that reproduces the bug where hash aggregation
produces duplicate group keys after spilling to disk.
The root cause: update_merged_stream() sets GroupOrdering::Full but does not
recreate group_values, leaving GroupValuesColumn<false> (vectorized_intern)
active. Under hash collisions, vectorized_intern produces non-monotonic group
indices, causing GroupOrderingFull to prematurely emit groups.
Introduces a new force_hash_partial_collisions feature that truncates hashes
to 5 bits (32 distinct values) instead of all zeros. Full collisions
paradoxically do not trigger the bug because all rows take the same code path
producing monotonic indices. Partial collisions create the necessary mix of
fast-path and slow-path rows.
## Which issue does this PR close?
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- Closes #20724
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