edponce commented on a change in pull request #11257:
URL: https://github.com/apache/arrow/pull/11257#discussion_r718676254



##########
File path: cpp/src/arrow/compute/kernels/aggregate_basic.cc
##########
@@ -754,6 +839,30 @@ void RegisterScalarAggregateBasic(FunctionRegistry* 
registry) {
                aggregate::CountInit, func.get());
   DCHECK_OK(registry->AddFunction(std::move(func)));
 
+  func = std::make_shared<ScalarAggregateFunction>(
+      "count_distinct", Arity::Unary(), &count_distinct_doc, 
&default_count_options);
+
+  // Takes any input, outputs int64 scalar
+  aggregate::AddCountDistinctKernel<Int8Type>(int8(), func.get());
+  aggregate::AddCountDistinctKernel<Int16Type>(int16(), func.get());
+  aggregate::AddCountDistinctKernel<Int32Type>(int32(), func.get());
+  aggregate::AddCountDistinctKernel<Date32Type>(date32(), func.get());
+  aggregate::AddCountDistinctKernel<Int64Type>(int64(), func.get());

Review comment:
       I support trying to use `Type::PhysicalType` and if you need generator 
dispatcher...well instead of making a custom one you could add an `Exec` method 
that redirects to `ExecArray` or `ExecScalar` and then you could use the 
existing GDs.




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