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GH-50869: [C++][Compute] Tighten coalesce exact dispatch for decimal varargs#50870

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GH-50869: [C++][Compute] Tighten coalesce exact dispatch for decimal varargs #50870

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662a932
GH-50869: [C++][Compute] Tighten coalesce exact dispatch for decimal …
zanmato1984 Jun 5, 2026
f051dd3
GH-50869: [C++][Compute] Address coalesce review feedback
zanmato1984 Sep 2, 2026
4c8b41c
Merge branch 'main' into codex/coalesce-exact-dispatch-decimal
zanmato1984 Sep 3, 2026
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64 changes: 64 additions & 0 deletions cpp/src/arrow/compute/expression_test.cc
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Original file line numberDiff line numberDiff line change
Expand Up@@ -938,6 +938,70 @@ TEST(Expression, BindWithImplicitCastsForCaseWhenOnDecimal) {
/*bound_out=*/nullptr, *exciting_schema);
}

TEST(Expression, BindWithImplicitCastsForCoalesceOnDecimal) {
auto exciting_schema = schema(
{field("dec128_3_2", decimal128(3, 2)), field("dec128_4_1", decimal128(4, 1)),
field("dec128_4_2", decimal128(4, 2)), field("dec128_4_3", decimal128(4, 3)),
field("dec256_3_2", decimal256(3, 2)), field("dec256_4_1", decimal256(4, 1))});

ExpectBindsTo(call("coalesce", {field_ref("dec128_3_2"), field_ref("dec128_4_2")}),
call("coalesce", {cast(field_ref("dec128_3_2"), decimal128(4, 2)),
field_ref("dec128_4_2")}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_4_2"), field_ref("dec128_3_2")}),
call("coalesce", {field_ref("dec128_4_2"),
cast(field_ref("dec128_3_2"), decimal128(4, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_4_1"), field_ref("dec128_3_2")}),
call("coalesce", {cast(field_ref("dec128_4_1"), decimal128(5, 2)),
cast(field_ref("dec128_3_2"), decimal128(5, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_3_2"), field_ref("dec128_4_1")}),
call("coalesce", {cast(field_ref("dec128_3_2"), decimal128(5, 2)),
cast(field_ref("dec128_4_1"), decimal128(5, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_3_2"), field_ref("dec128_4_3")}),
call("coalesce", {cast(field_ref("dec128_3_2"), decimal128(4, 3)),
field_ref("dec128_4_3")}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_4_3"), field_ref("dec128_3_2")}),
call("coalesce", {field_ref("dec128_4_3"),
cast(field_ref("dec128_3_2"), decimal128(4, 3))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_3_2"), field_ref("dec256_3_2")}),
call("coalesce", {cast(field_ref("dec128_3_2"), decimal256(3, 2)),
field_ref("dec256_3_2")}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec256_3_2"), field_ref("dec128_3_2")}),
call("coalesce", {field_ref("dec256_3_2"),
cast(field_ref("dec128_3_2"), decimal256(3, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec256_4_1"), field_ref("dec128_3_2")}),
call("coalesce", {cast(field_ref("dec256_4_1"), decimal256(5, 2)),
cast(field_ref("dec128_3_2"), decimal256(5, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
ExpectBindsTo(call("coalesce", {field_ref("dec128_3_2"), field_ref("dec256_4_1")}),
call("coalesce", {cast(field_ref("dec128_3_2"), decimal256(5, 2)),
cast(field_ref("dec256_4_1"), decimal256(5, 2))}),
/*bound_out=*/nullptr, *exciting_schema);
}

TEST(Expression, ExecuteCoalesceOnMixedDecimalTypes) {
ASSERT_OK_AND_ASSIGN(
auto input, StructArray::Make(
ArrayVector{ArrayFromJSON(decimal128(3, 2), R"(["1.23", null])"),
ArrayFromJSON(decimal128(4, 3), R"([null, "2.345"])")},
std::vector<std::string>{"left", "right"}));
Schema input_schema(input->type()->fields());
auto expr = call("coalesce", {field_ref("left"), field_ref("right")});

ASSERT_OK_AND_ASSIGN(expr, expr.Bind(input_schema));
ASSERT_OK_AND_ASSIGN(auto actual,
ExecuteScalarExpression(expr, input_schema, Datum(input)));

AssertDatumsEqual(actual, ArrayFromJSON(decimal128(4, 3), R"(["1.230", "2.345"])"));
}

TEST(Expression, BindNestedCall) {
auto expr = add(field_ref("a"),
call("subtract", {call("multiply", {field_ref("b"), field_ref("c")}),
Expand Down
16 changes: 16 additions & 0 deletions cpp/src/arrow/compute/kernel.cc
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Expand Up@@ -519,6 +519,22 @@ std::shared_ptr<MatchConstraint> DecimalsHaveSameScale() {
return instance;
}

std::shared_ptr<MatchConstraint> AllTypesAreIdenticalFrom(size_t first_type_index) {
return MatchConstraint::Make(
[first_type_index](const std::vector<TypeHolder>& types) -> bool {
DCHECK_LT(first_type_index, types.size());
return std::all_of(types.begin() + first_type_index + 1, types.end(),
[&types, first_type_index](const TypeHolder& type) {
return type == types[first_type_index];
});
});
}

std::shared_ptr<MatchConstraint> AllTypesAreIdentical() {
static auto instance = AllTypesAreIdenticalFrom(/*first_type_index=*/0);
return instance;
}

// ----------------------------------------------------------------------
// KernelSignature

Expand Down
7 changes: 7 additions & 0 deletions cpp/src/arrow/compute/kernel.h
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Expand Up@@ -365,6 +365,13 @@ class ARROW_EXPORT MatchConstraint {
/// \brief Constraint that all input types are decimal types and have the same scale.
ARROW_EXPORT std::shared_ptr<MatchConstraint> DecimalsHaveSameScale();

/// \brief Constraint that all input types are identical.
ARROW_EXPORT std::shared_ptr<MatchConstraint> AllTypesAreIdentical();

/// \brief Constraint that all input types starting at first_type_index are identical.
ARROW_EXPORT std::shared_ptr<MatchConstraint> AllTypesAreIdenticalFrom(
size_t first_type_index);

/// \brief Holds the input types, optional match constraint and output type of the kernel.
///
/// VarArgs functions with minimum N arguments should pass up to N input types to be
Expand Down
17 changes: 17 additions & 0 deletions cpp/src/arrow/compute/kernel_test.cc
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Expand Up@@ -341,6 +341,23 @@ TEST(MatchConstraint, DecimalsHaveSameScale) {
decimal128(precision, scale + 1)}));
}

TEST(MatchConstraint, AllTypesAreIdentical) {
auto c = AllTypesAreIdentical();
constexpr int32_t precision = 12, scale = 2;
ASSERT_TRUE(c->Matches({int8()}));
ASSERT_TRUE(c->Matches({decimal128(precision, scale), decimal128(precision, scale),
decimal128(precision, scale)}));
ASSERT_FALSE(
c->Matches({decimal128(precision, scale), decimal128(precision + 1, scale)}));
ASSERT_FALSE(
c->Matches({decimal128(precision, scale), decimal128(precision, scale + 1)}));
ASSERT_FALSE(c->Matches({decimal128(precision, scale), decimal256(precision, scale)}));

auto skip_first = AllTypesAreIdenticalFrom(/*first_type_index=*/1);
ASSERT_TRUE(skip_first->Matches({boolean(), utf8(), utf8()}));
ASSERT_FALSE(skip_first->Matches({boolean(), utf8(), binary()}));
}

// ----------------------------------------------------------------------
// KernelSignature

Expand Down
25 changes: 8 additions & 17 deletions cpp/src/arrow/compute/kernels/scalar_if_else.cc
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Expand Up@@ -1494,18 +1494,6 @@ struct CaseWhenFunction : ScalarFunction {
if (auto kernel = DispatchExactImpl(this, *types)) return kernel;
return arrow::compute::detail::NoMatchingKernel(this, *types);
}

// For case_when exact dispatch, all value arguments must have identical DataType.
static std::shared_ptr<MatchConstraint> AllValueTypesMatchConstraint() {
static auto constraint =
MatchConstraint::Make([](const std::vector<TypeHolder>& types) -> bool {
DCHECK_GE(types.size(), 2);
return std::all_of(
types.begin() + 2, types.end(),
[&types](const TypeHolder& type) { return type == types[1]; });
});
return constraint;
}
};

// Implement a 'case when' (SQL)/'select' (NumPy) function for any scalar conditions
Expand DownExpand Up@@ -2793,9 +2781,10 @@ void AddNestedCaseWhenKernels(const std::shared_ptr<CaseWhenFunction>& scalar_fu
}

void AddCoalesceKernel(const std::shared_ptr<ScalarFunction>& scalar_function,
detail::GetTypeId get_id, ArrayKernelExec exec) {
detail::GetTypeId get_id, ArrayKernelExec exec,
std::shared_ptr<MatchConstraint> constraint = nullptr) {
ScalarKernel kernel(KernelSignature::Make({InputType(get_id.id)}, FirstType,
/*is_varargs=*/true),
/*is_varargs=*/true, std::move(constraint)),
exec);
kernel.null_handling = NullHandling::COMPUTED_PREALLOCATE;
kernel.mem_allocation = MemAllocation::PREALLOCATE;
Expand DownExpand Up@@ -2911,7 +2900,7 @@ void RegisterScalarIfElse(FunctionRegistry* registry) {
{
auto func = std::make_shared<CaseWhenFunction>(
"case_when", Arity::VarArgs(/*min_args=*/2), case_when_doc);
auto all_value_types_match = CaseWhenFunction::AllValueTypesMatchConstraint();
auto all_value_types_match = AllTypesAreIdenticalFrom(/*first_type_index=*/1);
AddPrimitiveCaseWhenKernels(func, NumericTypes(), all_value_types_match);
AddPrimitiveCaseWhenKernels(func, TemporalTypes(), all_value_types_match);
AddPrimitiveCaseWhenKernels(func, IntervalTypes(), all_value_types_match);
Expand All@@ -2938,8 +2927,10 @@ void RegisterScalarIfElse(FunctionRegistry* registry) {
AddPrimitiveCoalesceKernels(func, {boolean(), null(), float16()});
AddCoalesceKernel(func, Type::FIXED_SIZE_BINARY,
CoalesceFunctor<FixedSizeBinaryType>::Exec);
AddCoalesceKernel(func, Type::DECIMAL128, CoalesceFunctor<FixedSizeBinaryType>::Exec);
AddCoalesceKernel(func, Type::DECIMAL256, CoalesceFunctor<FixedSizeBinaryType>::Exec);
AddCoalesceKernel(func, Type::DECIMAL128, CoalesceFunctor<FixedSizeBinaryType>::Exec,
AllTypesAreIdentical());
AddCoalesceKernel(func, Type::DECIMAL256, CoalesceFunctor<FixedSizeBinaryType>::Exec,
AllTypesAreIdentical());
for (const auto& ty : BaseBinaryTypes()) {
AddCoalesceKernel(func, ty, GenerateTypeAgnosticVarBinaryBase<CoalesceFunctor>(ty));
}
Expand Down
33 changes: 33 additions & 0 deletions cpp/src/arrow/compute/kernels/scalar_if_else_test.cc
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Expand Up@@ -3693,8 +3693,26 @@ TEST(TestCoalesce, DispatchBest) {
CheckDispatchBest("coalesce", {int32(), decimal128(3, 2)},
{decimal128(12, 2), decimal128(12, 2)});
CheckDispatchBest("coalesce", {float32(), decimal128(3, 2)}, {float64(), float64()});
CheckDispatchBest("coalesce", {decimal128(3, 2), decimal128(4, 2)},
{decimal128(4, 2), decimal128(4, 2)});
CheckDispatchBest("coalesce", {decimal128(4, 2), decimal128(3, 2)},
{decimal128(4, 2), decimal128(4, 2)});
CheckDispatchBest("coalesce", {decimal128(4, 1), decimal128(3, 2)},
{decimal128(5, 2), decimal128(5, 2)});
CheckDispatchBest("coalesce", {decimal128(3, 2), decimal128(4, 1)},
{decimal128(5, 2), decimal128(5, 2)});
CheckDispatchBest("coalesce", {decimal128(3, 2), decimal128(4, 3)},
{decimal128(4, 3), decimal128(4, 3)});
CheckDispatchBest("coalesce", {decimal128(4, 3), decimal128(3, 2)},
{decimal128(4, 3), decimal128(4, 3)});
CheckDispatchBest("coalesce", {decimal128(3, 2), decimal256(3, 2)},
{decimal256(3, 2), decimal256(3, 2)});
CheckDispatchBest("coalesce", {decimal256(3, 2), decimal128(3, 2)},

@pitroupitrouSep 1, 2026

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What about e.g. {decimal256(4, 1), decimal128(3, 2)}?

@zanmato1984zanmato1984Sep 2, 2026

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Good catch. I added this case in both argument orders. The common type is decimal256(5, 2), and the binding tests verify that both inputs are cast to it.

{decimal256(3, 2), decimal256(3, 2)});
CheckDispatchBest("coalesce", {decimal256(4, 1), decimal128(3, 2)},
{decimal256(5, 2), decimal256(5, 2)});
CheckDispatchBest("coalesce", {decimal128(3, 2), decimal256(4, 1)},
{decimal256(5, 2), decimal256(5, 2)});
CheckDispatchBest("coalesce", {timestamp(TimeUnit::SECOND), date32()},
{timestamp(TimeUnit::SECOND), timestamp(TimeUnit::SECOND)});
CheckDispatchBest("coalesce", {timestamp(TimeUnit::SECOND), timestamp(TimeUnit::MILLI)},
Expand All@@ -3710,6 +3728,21 @@ TEST(TestCoalesce, DispatchBest) {
{large_binary(), large_binary()});
}

TEST(TestCoalesce, DispatchExact) {
CheckDispatchExact("coalesce", {decimal128(3, 2), decimal128(3, 2)});
CheckDispatchExact("coalesce", {decimal256(3, 2), decimal256(3, 2)});
CheckDispatchExactFails("coalesce", {decimal128(3, 2), decimal128(4, 2)});
CheckDispatchExactFails("coalesce", {decimal128(4, 2), decimal128(3, 2)});
CheckDispatchExactFails("coalesce", {decimal128(4, 1), decimal128(3, 2)});
CheckDispatchExactFails("coalesce", {decimal128(3, 2), decimal128(4, 1)});
CheckDispatchExactFails("coalesce", {decimal128(3, 2), decimal128(4, 3)});
CheckDispatchExactFails("coalesce", {decimal128(4, 3), decimal128(3, 2)});
CheckDispatchExactFails("coalesce", {decimal128(3, 2), decimal256(3, 2)});
CheckDispatchExactFails("coalesce", {decimal256(3, 2), decimal128(3, 2)});
CheckDispatchExactFails("coalesce", {decimal256(4, 1), decimal128(3, 2)});
CheckDispatchExactFails("coalesce", {decimal128(3, 2), decimal256(4, 1)});
}

template <typename Type>
class TestChooseNumeric : public ::testing::Test {};
template <typename Type>
Expand Down
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