Improve predicate pushdown coverage in cudf-polars

[Matt711]

Description

A predicate that is pushed down to a Scan in the logical plan does not imply it's used in the libcudf parquet reader for row group pruning based on statistics and the post read filter in libcudf. The predicate must first be translatable to libcudf AST. If it is not, then we use the predicate to filter (post read) at the end of Scan execution. This PR makes two improvements

1. For conjunctive predicates, we can push down conjuncts that we do support converting to AST (think "partial predicate pushdown")
2. Allow boolean functions is_null and is_not_null to pass through AST conversion because they are supported. I'll do another audit here to see if there are anymore we can allow through.

Checklist

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

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 5139e7d and 7fe3967.

📒 Files selected for processing (4)

• python/cudf_polars/cudf_polars/dsl/ir.py
• python/cudf_polars/cudf_polars/dsl/to_ast.py
• python/cudf_polars/tests/dsl/test_to_ast.py
• python/cudf_polars/tests/test_parquet_filters.py

🚧 Files skipped from review as they are similar to previous changes (4)

• python/cudf_polars/tests/test_parquet_filters.py
• python/cudf_polars/tests/dsl/test_to_ast.py
• python/cudf_polars/cudf_polars/dsl/ir.py
• python/cudf_polars/cudf_polars/dsl/to_ast.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Improved Parquet filtering for partially supported conditions, applying remaining checks after data is read.
  • Added reliable null-check filtering for supported columns and combined filter expressions.

• Bug Fixes

  • Prevented unsupported filters on nested structures from being pushed to the Parquet reader.
  • Avoided unnecessary post-read filtering when all conditions are handled during reading.
  • Expanded coverage for string matching, numeric, null-check, and compound filter combinations.

Walkthrough

Parquet filter conversion now returns reader filters and an optional residual expression. Parquet scans apply residual expressions after reading. Nested STRUCT and LIST null checks are excluded from pushdown. Tests cover full, partial, and unsupported predicate conversion.

Changes

Parquet predicate pushdown

Layer / File(s)	Summary
Filter conversion and residual construction python/cudf_polars/cudf_polars/dsl/to_ast.py	The converter supports null checks on supported columns, rejects nested STRUCT and LIST operations, splits conjunctions, and returns reader filters with optional residual expressions.
Scan residual predicate application python/cudf_polars/cudf_polars/dsl/ir.py	Scan.do_evaluate retains residual predicates after Parquet pushdown and applies them after reading. The fast-return path runs only when no residual predicate remains.
Predicate conversion and scan validation python/cudf_polars/tests/dsl/test_to_ast.py, python/cudf_polars/tests/test_parquet_filters.py	Tests cover null checks, nested columns, conjunction splitting, unsupported disjunctions, and combined string, numeric, and null predicates.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to 7fe39

The PR improves predicate pushdown by splitting supported conjuncts and allowing null checks in parquet filtering. It is mergeable with owner awareness, but the new path lacks a required performance benchmark, so efficiency regressions could go undetected.

Suggested reviewers: nirandaperera

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains partial predicate pushdown and support for null checks in cudf-polars.
Title check	✅ Passed	The title clearly summarizes the main change: improved predicate pushdown coverage in cudf-polars.
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

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[TomAugspurger]

Looks nice. Might have an issue with struct columns though:

FAILED py-polars/tests/unit/io/test_delta.py::test_scan_delta_predicate_pushdown_struct_is_not_null - ValueError: CUDF failure at: /__w/cudf/cudf/cpp/src/io/parquet/predicate_pushdown.cpp:162: Column chunk with schema index 1 not found in source 0

https://github.com/rapidsai/cudf/actions/runs/28878719201/job/85673148030?pr=23143#step:13:2937

[vyasr]

@Matt711 do you want to pick this back up?

[wence-]

Tiny nit

[wence-]

Nit: this destroys the dag structure of the expression since it re-traverses all children even if they are already seen.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@python/cudf_polars/cudf_polars/dsl/to_ast.py`:
- Around line 298-347: Add unit benchmarks covering fully pushable, partially
pushable, and unsupported predicates through to_parquet_filter, measuring
predicate conversion and the subsequent scan path separately. Use representative
expressions for each category and follow the project’s existing benchmark
conventions.

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• Push a commit to this branch (recommended)
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Configuration used: Path: .coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 3df675e and 896aade.

📒 Files selected for processing (4)

• python/cudf_polars/cudf_polars/dsl/ir.py
• python/cudf_polars/cudf_polars/dsl/to_ast.py
• python/cudf_polars/tests/dsl/test_to_ast.py
• python/cudf_polars/tests/test_parquet_filters.py

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[vyasr]

/ok to test 896aade

[vyasr]

/ok to test 7fe3967

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[vyasr]

This PR was already approved before the NVIDIA org transition so I'm just rubber-stamping to satisfy the new merge requirements.

[vyasr]

/merge