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]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 7ddbae9 and 75ac2ba.

📒 Files selected for processing (5)

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

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Improved Parquet filter handling so more predicates can be pushed down during reads, including partial support when only some conditions can be applied early.
  • Added support for null-check filters in Parquet pushdown and better handling of combined filter expressions.

• Bug Fixes

  • Reduced unnecessary post-read filtering by more accurately detecting when a Parquet filter is fully applied.
  • Expanded test coverage for string-based and compound filter cases to prevent regressions.

Walkthrough

The PR changes to_parquet_filter to return a tuple of the converted expression and an "exact" boolean instead of just an optional expression. Consumers in ir.py and actor_graph/io.py now use this exactness flag to decide fallback/masking behavior. Tests were added/extended accordingly.

Changes

Parquet filter exactness

Layer / File(s)	Summary
to_parquet_filter conversion and exactness logic python/cudf_polars/cudf_polars/dsl/to_ast.py	Relaxes parquet BooleanFunction handling for IsNull/IsNotNull on columns, adds _extract_conjuncts and _to_parquet_filter helpers, and reworks to_parquet_filter to return (expr, exact) by attempting whole-predicate conversion then per-conjunct fallback.
Scan and streaming read node consumption python/cudf_polars/cudf_polars/dsl/ir.py, python/cudf_polars/cudf_polars/streaming/actor_graph/io.py	Scan.do_evaluate and make_rapidsmpf_read_parquet_node capture filters_exact and use it instead of filter presence to decide whether post-read masking or pushdown fallback is needed.
Tests for exactness and new filter expressions python/cudf_polars/tests/dsl/test_to_ast.py, python/cudf_polars/tests/test_parquet_filters.py	Adds parametrized tests for to_parquet_filter exactness on null-check and conjunction/disjunction predicates, and extends parquet filter test expressions with string-predicate combinations.

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

Possibly related PRs

• rapidsai/cudf#21460: Both PRs modify Parquet predicate handling in make_rapidsmpf_read_parquet_node, changing pushdown/fallback decision logic.

Suggested labels: 3 - Ready for Review

Suggested reviewers: madsbk

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: improved predicate pushdown coverage in cudf-polars.
Description check	✅ Passed	The description directly matches the changeset and explains the predicate pushdown improvements.
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

🧪 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.