Optimize hash full join

[PointKernel]

Description

This PR tracks matched right-table rows during hash full join retrieval, eliminating output-sized temporary storage and a subsequent scatter when finding unmatched rows. It also reserves space for the right-side complement and fixes full_join_size when the left table is empty.

Checklist

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

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[PointKernel]

/ok to test 1973121

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 41dc4d9c-c136-43f9-b2bd-82fbf0b3bf77

📥 Commits

Reviewing files that changed from the base of the PR and between a70dbd8 and b53a5ab.

📒 Files selected for processing (1)

• cpp/src/join/join_utils.cu

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

• cpp/src/join/join_utils.cu

---

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved full-join handling to consistently include unmatched rows from the right-hand table.
  • Corrected full-join output sizing when the left-hand table is empty.
  • Improved match tracking and output finalization while preserving existing behavior for other join types.

• Tests

  • Added coverage confirming that full joins with an empty left-hand table return all right-hand rows with correctly sized index results.

Walkthrough

Full hash joins now record matched right-table rows in a device buffer. Full-join sizing and finalization use this buffer to count and append unmatched right rows. Related join paths pass the updated finalization argument.

Changes

Full-join match tracking

Layer / File(s)	Summary
Probe-time right-row match tracking cpp/src/join/hash_join/dispatch.cuh, cpp/src/join/hash_join/retrieve_impl.cuh	Full joins allocate right-row match flags. Probe output marks matched rows and passes the flags to sizing and finalization.
Match-based full-join sizing cpp/src/join/hash_join/full_join_size_impl.cu, cpp/src/join/hash_join/size_impl.cuh	Full-join sizing counts unmatched right rows from match flags and removes the unused memory-resource parameter.
Full-join finalization and integration cpp/src/join/join_common_utils.hpp, cpp/src/join/join_utils.cu, cpp/src/join/conditional_join.cu, cpp/src/join/mixed_join.cu, cpp/tests/join/join_tests.cpp	Finalization accepts optional match flags, validates their size, and compacts unmatched rows. Related join paths pass the new argument, and the empty-left test checks output sizes.

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

Possibly related PRs

• rapidsai/cudf#23012: Modifies shared finalize_full_join and full-join unmatched-right-row handling.
• rapidsai/cudf#23398: Shares full-join and mixed-join integration changes.

Suggested reviewers: qbacpey

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% 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
Title check	✅ Passed	The title clearly summarizes the main change: optimizing hash full joins.
Description check	✅ Passed	The description accurately explains the hash full-join optimization, right-row match tracking, storage reduction, and empty-left fix.
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

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 3

🧹 Nitpick comments (1)

cpp/tests/join/join_tests.cpp (1)

1717-1723: 📐 Maintainability & Code Quality | 🔵 Trivial

Verify the required code-owner approvals.

This file matches cpp/**/*.cpp. Obtain at least two approvals from cudf-cpp-codeowners before merge.

As per coding guidelines, changes to libcudf C++ files require at least two approvals from cudf-cpp-codeowners.

🤖 Prompt for 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.

In `@cpp/tests/join/join_tests.cpp` around lines 1717 - 1723, Obtain at least two
approvals from the cudf-cpp-codeowners group for the changes in the C++ test
file before merging.

Source: Coding guidelines

🤖 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 `@cpp/src/join/hash_join/full_join_size_impl.cu`:
- Around line 20-24: Update the device lambda predicate passed to count_if in
the full join size implementation to explicitly declare its return type as bool,
while preserving its existing is_matched == 0 condition.
- Around line 50-51: Update the right_matches allocation in the full-join size
implementation to use cudf::get_current_device_resource_ref() instead of mr. If
mr is no longer referenced, remove it from the internal interface or explicitly
mark it unused while preserving the existing allocation behavior.

In `@cpp/src/join/join_common_utils.hpp`:
- Around line 75-80: Reorder finalize_full_join so right_matches precedes stream
and mr, remove its default argument, and update the Doxygen parameter order. In
cpp/src/join/join_utils.cu:91-92 and cpp/src/join/mixed_join.cu and
cpp/src/join/conditional_join.cu, pass {} explicitly for no-flag calls; in
cpp/src/join/hash_join/retrieve_impl.cuh:216-218, update the caller to the same
argument order.

---

Nitpick comments:
In `@cpp/tests/join/join_tests.cpp`:
- Around line 1717-1723: Obtain at least two approvals from the
cudf-cpp-codeowners group for the changes in the C++ test file before merging.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

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

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: aae2eb87-9bef-4a0a-a544-63a0a1210c35

📥 Commits

Reviewing files that changed from the base of the PR and between 5ba02d1 and 1973121.

📒 Files selected for processing (6)

• cpp/src/join/hash_join/dispatch.cuh
• cpp/src/join/hash_join/full_join_size_impl.cu
• cpp/src/join/hash_join/retrieve_impl.cuh
• cpp/src/join/join_common_utils.hpp
• cpp/src/join/join_utils.cu
• cpp/tests/join/join_tests.cpp

[PointKernel]

NVBench comparison: hash full join

Reference: PR base at 5ba02d13606e5a3f43eddb3566d43124e1c4950c; comparison: PR head at 1973121bf91afa61364c620b4a3d8a2845b16919. Both were built from the same isolated worktree and CMake build directory. Hardware: NVIDIA GH200 480GB, CUDA 13.3, driver 580.95.05.

Coverage: full_join spans INT32, INT64, FLOAT32, FLOAT64, STRING, LIST, and STRUCT; nullable/non-nullable inputs, 1–3 keys, and 10k/100k table sizes. high_multiplicity_full_join covers INT32 hash joins, nullable/non-nullable inputs, both null modes, 1–3 keys, 10k/100k table sizes, and multiplicities 100 and 1,000. Meaningful result: 24 faster, 0 slower, and 174 within the 5%/noise threshold.

Command: nvbench/scripts/nvbench_compare.py --threshold-diff 0.05 <base> <head>. Ref is the PR base and Cmp is this change. The table contains rows with at least a 5% absolute difference. The final “Failure” count means a difference above measured noise in either direction, including FAST; it is not a regression count.

full_join

high_multiplicity_full_join

[0] NVIDIA GH200 480GB

Nullable	NullEquality	DataType	Algorithm	num_keys	left_size	right_size	multiplicity	skip_large_sizes	Ref Time	Ref Noise	Cmp Time	Cmp Noise	Diff	%Diff	Status
0	NULLS_EQUAL	INT32	HASH	1	100000	10000	100	1	1.448 ms	0.96%	1.317 ms	1.36%	-131.465 us	-9.08%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	2	100000	10000	100	1	1.667 ms	1.00%	1.539 ms	0.85%	-128.306 us	-7.70%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	3	100000	10000	100	1	1.940 ms	0.59%	1.809 ms	0.68%	-130.922 us	-6.75%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	1	100000	10000	1000	1	10.338 ms	0.43%	9.006 ms	0.42%	-1332.040 us	-12.88%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	2	100000	10000	1000	1	11.861 ms	0.37%	10.525 ms	0.36%	-1335.935 us	-11.26%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	3	100000	10000	1000	1	13.699 ms	0.35%	12.369 ms	0.37%	-1330.212 us	-9.71%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	1	100000	100000	1000	1	10.720 ms	0.46%	9.999 ms	0.38%	-721.580 us	-6.73%	🟢 FAST
0	NULLS_EQUAL	INT32	HASH	2	100000	100000	1000	1	13.126 ms	0.41%	12.378 ms	0.30%	-748.042 us	-5.70%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	1	100000	10000	100	1	1.466 ms	0.93%	1.341 ms	0.94%	-125.012 us	-8.53%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	2	100000	10000	100	1	1.683 ms	0.71%	1.557 ms	0.78%	-125.930 us	-7.48%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	3	100000	10000	100	1	1.957 ms	0.60%	1.832 ms	0.72%	-124.322 us	-6.35%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	1	100000	10000	1000	1	10.397 ms	0.50%	9.042 ms	0.43%	-1355.340 us	-13.04%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	2	100000	10000	1000	1	11.926 ms	0.46%	10.559 ms	0.36%	-1367.305 us	-11.46%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	3	100000	10000	1000	1	13.784 ms	0.34%	12.407 ms	0.32%	-1376.847 us	-9.99%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	1	100000	100000	1000	1	10.778 ms	0.50%	10.031 ms	0.43%	-747.009 us	-6.93%	🟢 FAST
0	NULLS_UNEQUAL	INT32	HASH	2	100000	100000	1000	1	13.171 ms	0.32%	12.411 ms	0.28%	-760.036 us	-5.77%	🟢 FAST
1	NULLS_EQUAL	INT32	HASH	1	100000	10000	100	1	25.294 ms	0.50%	23.322 ms	0.55%	-1972.686 us	-7.80%	🟢 FAST
1	NULLS_EQUAL	INT32	HASH	3	100000	10000	100	1	2.467 ms	0.77%	2.339 ms	0.77%	-128.439 us	-5.21%	🟢 FAST
1	NULLS_EQUAL	INT32	HASH	1	100000	10000	1000	1	34.285 ms	0.49%	31.024 ms	0.73%	-3261.818 us	-9.51%	🟢 FAST
1	NULLS_EQUAL	INT32	HASH	2	100000	10000	1000	1	13.797 ms	0.30%	12.276 ms	0.20%	-1520.913 us	-11.02%	🟢 FAST
1	NULLS_EQUAL	INT32	HASH	3	100000	10000	1000	1	15.557 ms	0.39%	14.295 ms	0.43%	-1262.160 us	-8.11%	🟢 FAST
1	NULLS_UNEQUAL	INT32	HASH	1	100000	10000	100	1	1.266 ms	0.96%	1.189 ms	0.91%	-77.770 us	-6.14%	🟢 FAST
1	NULLS_UNEQUAL	INT32	HASH	1	100000	10000	1000	1	8.380 ms	0.38%	7.665 ms	0.30%	-715.242 us	-8.54%	🟢 FAST
1	NULLS_UNEQUAL	INT32	HASH	2	100000	10000	1000	1	8.776 ms	0.34%	8.335 ms	0.32%	-441.159 us	-5.03%	🟢 FAST

Summary

• Total Matches: 198
  • Pass (diff <= min_noise): 79
  • Unknown (infinite noise): 0
  • Failure (diff > min_noise): 119