Support zero-column operands in cross_join

[madsbk]

Description

Previously, cross_join threw "Left table is empty" or "Right table is empty" when either operand had zero columns. Zero-column operands with non-zero row counts are now valid. The result has left.num_rows() * right.num_rows() rows and left.num_columns() + right.num_columns() columns.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 428daff and a2dfdaa.

📒 Files selected for processing (3)

• cpp/include/cudf/join/join.hpp
• python/cudf/cudf/core/join/join.py
• python/cudf/cudf/tests/reshape/test_merge.py

🚧 Files skipped from review as they are similar to previous changes (3)

• python/cudf/cudf/core/join/join.py
• cpp/include/cudf/join/join.hpp
• python/cudf/cudf/tests/reshape/test_merge.py

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Cross joins now document and properly handle zero-column inputs, preserving the expected rows = left_rows × right_rows and combined column count.
  • Cross joins now detect row-count overflow and raise std::overflow_error when the computed result size exceeds supported limits.
  • Cross merges (how="cross") now retain correct row expansion for column-less operands, matching pandas behavior.
• Tests
  • Expanded cross-join and cross-merge test coverage for zero-column and overflow scenarios.
• Documentation
  • Updated cross_join API docs to reflect the exact output shape and overflow conditions.

Walkthrough

Cross joins now support zero-column operands while preserving the product of input row counts. Native code validates output-size overflow and passes explicit row counts to table construction. Python cross merges preserve row counts through an explicit index.

Changes

Cross Join Zero-Column Support

Layer / File(s)	Summary
Native cross-join row-count handling cpp/include/cudf/join/join.hpp, cpp/src/join/cross_join.cu, cpp/tests/join/cross_join_tests.cpp	Documents cross-join output dimensions, permits zero-column inputs, checks row-count overflow, constructs explicit-sized outputs, and tests row-count preservation.
Python cross-merge row preservation python/cudf/cudf/core/join/join.py, python/cudf/cudf/tests/reshape/test_merge.py	Passes explicit operand row counts to libcudf, restores the index for zero-column results, and compares cross-merge results with pandas.

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

Possibly related PRs

• rapidsai/cudf#22765: Adds related support for preserving and transporting row counts in zero-column tables.

Suggested labels: bug

Suggested reviewers: bdice, mhaseeb123

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: allowing zero-column operands in cross_join.
Description check	✅ Passed	The description matches the changeset and explains the new zero-column cross_join behavior.
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.

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• Create PR with unit tests

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[PointKernel]

C++ changes look good to me

[mhaseeb123]

Approving C++. Thanks for the contribution

[madsbk]

/merge