Improve Parquet writer struct test

[mhaseeb123]

Description

Read back the struct column table in the parquet_writer_test.cpp and verify it round-trip perfectly.

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 1f29415 and 8b52d87.

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• cpp/tests/io/parquet_writer_test.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Tests
  • Improved Parquet struct round-trip validation by checking read results against the expected table.
  • Removed redundant coverage for struct fields with non-empty null values.

Walkthrough

The Parquet Struct test now checks the table returned by read_parquet. The separate StructWithNonEmptyNulls round-trip test was removed.

Changes

Parquet struct tests

Layer / File(s)	Summary
Struct round-trip validation cpp/tests/io/parquet_writer_test.cpp	The Struct test stores the Parquet read result and compares it with the expected table. The StructWithNonEmptyNulls test was removed.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 8b52d

This PR strengthens Parquet writer round-trip validation without changing production behavior. No actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: davidwendt, mattgara, pmattione-nvidia

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 9.09% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change to the Parquet writer struct test.
Description check	✅ Passed	The description directly explains the round-trip readback verification added to the Parquet writer struct test.
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.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@cpp/tests/io/parquet_writer_test.cpp`:
- Line 365: Add nullable-struct round-trip coverage in the relevant Parquet
writer tests for dictionary encoding and each changed delta encoding, replacing
the sole dictionary_policy::NEVER-only coverage with explicit policies that
exercise those paths. Also add the required unit benchmark for the row-aware
validity check, reusing existing test and benchmark helpers where available.

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📥 Commits

Reviewing files that changed from the base of the PR and between cc6fc85 and 1f29415.

📒 Files selected for processing (4)

• cpp/src/io/parquet/chunk_dict.cu
• cpp/src/io/parquet/page_enc.cu
• cpp/src/io/parquet/parquet_gpu.cuh
• cpp/tests/io/parquet_writer_test.cpp

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[mhaseeb123]

/merge