bkietz commented on code in PR #40687:
URL: https://github.com/apache/arrow/pull/40687#discussion_r1533941672
##########
cpp/src/arrow/compute/exec_test.cc:
##########
@@ -1425,6 +1426,33 @@ TEST(Ordering, IsSuborderOf) {
CheckOrdering(unordered, {true, true, true, true, true, true});
}
+static Status RegisterMyScalarPureFunction() {
+ const std::string name = "my_scalar_function";
+ auto func = std::make_shared<ScalarFunction>(name, Arity::Nullary(),
+ FunctionDoc::Empty(), nullptr);
+ auto func_exec = [](KernelContext* /*ctx*/, const ExecSpan& /*batch*/,
+ ExecResult* out) -> Status {
+ auto scalar = MakeScalar("I am a scalar");
+ ARROW_ASSIGN_OR_RAISE(auto arr_res, MakeArrayFromScalar(*scalar, 1));
+ out->value = std::move(arr_res->data());
+ return Status::OK();
+ };
+
+ ScalarKernel kernel({}, utf8(), func_exec);
+ ARROW_RETURN_NOT_OK(func->AddKernel(kernel));
+
+ auto registry = GetFunctionRegistry();
+ ARROW_RETURN_NOT_OK(registry->AddFunction(std::move(func)));
+ return Status::OK();
+}
+
+TEST(CallFunction, ScalarPureFunction) {
+ ASSERT_OK(RegisterMyScalarPureFunction());
+
+ EXPECT_THAT(CallFunction("my_scalar_function", ExecBatch({})),
+ ResultWith(MakeScalar("I am a scalar")));
+}
Review Comment:
Ah, I see. In that case, please add a comment
```c++
// Ensure that KernelExecutor's fast path for zero-length batches
// does not clobber scalar output for functions called with no arguments
```
it's worth noting that this is independent of function purity; for example a
function `query_start_time()` which always returns a Timestamp scalar of the
instant when the query started would be impure and would have run afoul of this
fast path too
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