[Awesome06]

Rationale for this change

Currently, Arrow compute supports comparing Utf8 with LargeUtf8 and comparing Utf8View with Utf8View, but attempting to compare a Utf8View array/scalar with either Utf8 or LargeUtf8 fails with:

ArrowNotImplementedError: Function 'equal' has no kernel matching input types (string, string_view)

In scalar_compare.cc, comparison operators (equal, not_equal, less, greater, etc.) enforce homogeneous symmetric kernel signatures ({ty, ty}) and rely on DispatchBest to resolve heterogeneous types via CommonBinary(). However, CommonBinary() did not previously recognize Type::STRING_VIEW or Type::BINARY_VIEW, returning nullptr and causing DispatchBest to fail.

When mixing string/binary view types with offset-based string/binary types, promoting to the view type (utf8_view or binary_view) is optimal:

1. ZeroCopyCastExec in scalar_cast_string.cc reuses the existing character data buffer directly as variadic buffer 0, only allocating 16-byte descriptors without copying string payload data. (Conversely, promoting to LargeUtf8 would require a deep copy of all string data into contiguous buffers).
2. For scalar-array comparisons (string_view_array == "literal"), only the scalar literal is converted (O(1) zero-allocation), leaving array memory completely untouched.

What changes are included in this PR?

1. cpp/src/arrow/compute/kernels/codegen_internal.cc:

   • Extended CommonBinary() to handle Type::STRING_VIEW and Type::BINARY_VIEW.
   • Promotes mixed string types to utf8_view(), and mixed binary/fixed-size types to binary_view().

2. cpp/src/arrow/compute/kernels/scalar_compare_test.cc:

   • Added CheckDispatchBest assertions for view <-> non-view binary and string combinations (utf8, large_utf8, utf8_view, binary, large_binary, binary_view, fixed_size_binary).
   • Added TEST(TestBinaryViewCompareKernel, MixedStringTypes) validating:
     • Short strings (<= 12 bytes, testing inline descriptors)
     • Long strings (> 12 bytes, testing out-of-line prefix + buffer lookup)
     • Sliced arrays (array.Slice(1, 2))
     • Empty arrays ([]) and all-null arrays ([null, null])
     • Array-scalar and scalar-array operations across equal, not_equal, less, and greater.

3. python/pyarrow/tests/test_compute.py:

   • Added test_compare_mixed_string_types parameterized across all 9 permutations of string, large_string, and string_view (including sliced and >12-byte strings).
   • Added test_compare_mixed_binary_types parameterized across all 9 permutations of binary, large_binary, and binary_view.

Are these changes tested?

Yes:

• C++ unit tests in arrow-compute-scalar-math-test (including TestCompareKernel.DispatchBest and TestBinaryViewCompareKernel.MixedStringTypes). All 594 tests in the test suite pass with 0 failures.
• PyArrow unit tests added to python/pyarrow/tests/test_compute.py.

Are there any user-facing changes?

Yes. Users can now directly compare Utf8View / BinaryView columns against Utf8, LargeUtf8, Binary, and LargeBinary columns or scalars in expressions and compute kernels without explicit casting or ArrowNotImplementedError.

Was AI used for this PR?

AI was used to diagnose the type promotion dispatch path, draft implementation code, and construct test cases under developer direction. The changes and tests were reviewed, compiled, and verified locally.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
• AI
• Not reviewed

• GitHub Issue: [C++] Implement equality comparison between Utf8View and other string types #54668

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[github-actions]

⚠️ GitHub issue #54668 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #54668 has been automatically assigned in GitHub to PR creator.