[C++][Compute] Conditional compute kernels (if_else, coalesce, case_when, choose) do not support string_view or binary_view #55255

Describe the bug, including details regarding any error messages, version, and platform.

The conditional compute kernels (if_else, coalesce, case_when, and choose) fail with arrow::Status::NotImplemented when passed utf8_view (string_view) or binary_view arrays or scalars.

While classic variable-length binary types (utf8, large_utf8, binary, large_binary) have full support across all conditional compute kernels, modern string/binary-view types are missing kernel registrations and fail type dispatch.

In cpp/src/arrow/compute/kernels/scalar_if_else.cc, the registration helpers only iterate over BaseBinaryTypes():

// if_else
AddBinaryIfElseKernels(func, BaseBinaryTypes());

// case_when
for (const auto& ty : BaseBinaryTypes()) {
  AddCaseWhenKernel(func, ty, GenerateTypeAgnosticVarBinaryBase<CaseWhenFunctor>(ty));
}

// coalesce
for (const auto& ty : BaseBinaryTypes()) {
  AddCoalesceKernel(func, ty, GenerateTypeAgnosticVarBinaryBase<CoalesceFunctor>(ty));
}

// choose
for (const auto& ty : BaseBinaryTypes()) {
  AddChooseKernel(func, ty, GenerateTypeAgnosticVarBinaryBase<ChooseFunctor>(ty));
}

BaseBinaryTypes() includes only utf8, large_utf8, binary, and large_binary. Neither StringViewType nor BinaryViewType is registered.

Furthermore, in cpp/src/arrow/compute/kernels/codegen_internal.cc, CommonBinary does not resolve view types and returns TypeHolder(nullptr).

Reproduction

The failure reproduces in C++ (and PyArrow):

TEST(TestIfElseKernel, StringViewSupport) {
  auto view_type = utf8_view();
  auto cond = ArrayFromJSON(boolean(), "[true, false, true]");
  auto left = ArrayFromJSON(view_type, "[\"apple\", \"banana\", \"cherry\"]");
  auto right = ArrayFromJSON(view_type, "[\"alpha\", \"beta\", \"gamma\"]");

  // Call if_else(cond, left, right)
  auto result = arrow::compute::IfElse(cond, left, right);
  ASSERT_OK(result.status());
}

Or in Python:

import pyarrow as pa
import pyarrow.compute as pc

cond = pa.array([True, False, True])
left = pa.array(["apple", "banana", "cherry"], type=pa.string_view())
right = pa.array(["alpha", "beta", "gamma"], type=pa.string_view())

pc.if_else(cond, left, right)

Expected:
Returns a string_view array: ["apple", "beta", "cherry"].

Actual:

ArrowNotImplementedError: Function 'if_else' has no kernel matching input types (boolean, string_view, string_view)

Similarly:

ArrowNotImplementedError: Function 'coalesce' has no kernel matching input types (string_view, string_view)
ArrowNotImplementedError: Function 'case_when' has no kernel matching input types (boolean, string_view, string_view)
ArrowNotImplementedError: Function 'choose' has no kernel matching input types (int8, string_view, string_view)

Reproduced on current main across all platforms. This is a type-matrix parity gap in Arrow C++ compute.

Suggested fix

Add zero-allocation direct dispatch for utf8_view and binary_view in cpp/src/arrow/compute/kernels/scalar_if_else.cc:

1. Because BinaryView / StringView values are represented as 16-byte view descriptors (arrow::util::BinaryViewType::c_type), conditional selection is extremely efficient:
   • The kernel can copy the selected 16-byte descriptors directly into the output array's data buffer.
   • The data buffers referenced by the input arrays can be gathered and appended to the output ArrayData::buffers without copying string payloads.
2. Register Type::STRING_VIEW and Type::BINARY_VIEW across if_else, coalesce, case_when, and choose.
3. Add tests in cpp/src/arrow/compute/kernels/scalar_if_else_test.cc verifying:
   • Inline short strings (<= 12 bytes) and out-of-line strings (> 12 bytes).
   • Sliced arrays with non-zero offsets (left.Slice(1, 2)).
   • Array-array, scalar-array, and array-scalar permutations.
   • All-null conditions and null input views.

Component(s)

C++, Compute