Preserve input order for grouped window evaluation

[rjzamora]

Description

Follow up to #23451
Fully addresses item (7) of #22032

Adds the necessary per-group sort operation to preserve input row order before evaluating input-order-sensitive over(...) expressions without an explicit order_by argument.

This allows streaming execution of grouped shift and cum_sum windows that rely on the incoming row order.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7ea279c and 6ca710a.

📒 Files selected for processing (1)

• python/cudf_polars/cudf_polars/streaming/actor_graph/over.py

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

• python/cudf_polars/cudf_polars/streaming/actor_graph/over.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved over() window evaluation in multi-partition streaming by preserving required input order via a stable, input-order-aware sort of stamped rows before computing window expressions.
  • Updated selection/lowering behavior to no longer fall back to an unsupported path for input-order-sensitive window expressions lacking order_by.
• Tests
  • Refreshed streaming SPMD coverage to verify correct multi-rank results for shift(1).over(...) and cum_sum().over(...) (no longer expecting an error), with deterministic global ordering.

Walkthrough

Changes

Multirank streaming over() evaluation now conditionally restores input order before evaluating applicable windows. The previous Select fallback is removed, and SPMD tests cover shift and cum_sum without order_by.

Multirank window ordering

Layer / File(s)	Summary
Stamped row ordering and evaluation python/cudf_polars/cudf_polars/streaming/actor_graph/over.py	Forward-shuffled stamped rows are optionally stably sorted by origin rank and chunk order before window evaluation.
Select lowering and multirank validation python/cudf_polars/cudf_polars/streaming/select.py, python/cudf_polars/tests/streaming/test_spmd.py	The multirank input-order window fallback is removed, and tests validate shift and cum_sum results without order_by.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• rapidsai/cudf#23206: Related multirank streaming handling for over() windows, including cum_sum fallback behavior.
• rapidsai/cudf#23306: Related changes to over() partitioning and fast-path decisions.
• rapidsai/cudf#23451: Related multirank support for implicit input-order window operations.

Suggested labels: improvement

Suggested reviewers: tomaugspurger

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: preserving input order for grouped window evaluation.
Description check	✅ Passed	The description is directly related to the changeset and describes the input-order-preserving window evaluation update.
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.

✨ Finishing Touches

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• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@python/cudf_polars/tests/streaming/test_spmd.py`:
- Around line 597-602: Strengthen the test around local_result in the SPMD
collection flow by asserting each rank owns the expected rows in the expected
local order before calling allgather_polars_dataframe. Add rank-aware ownership
checks using the test’s existing distributed context, then retain the global
sorted assertion for the gathered result.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 2318405c-4ef3-4cb4-b28c-26ad24613dc7

📥 Commits

Reviewing files that changed from the base of the PR and between f38653b and b4d9ed4.

📒 Files selected for processing (3)

• python/cudf_polars/cudf_polars/streaming/actor_graph/over.py
• python/cudf_polars/cudf_polars/streaming/select.py
• python/cudf_polars/tests/streaming/test_spmd.py

💤 Files with no reviewable changes (1)

• python/cudf_polars/cudf_polars/streaming/select.py

[rjzamora]

/merge