lidavidm commented on a change in pull request #11257:
URL: https://github.com/apache/arrow/pull/11257#discussion_r720615496



##########
File path: cpp/src/arrow/compute/kernels/aggregate_basic.cc
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@@ -121,6 +122,84 @@ Result<std::unique_ptr<KernelState>> 
CountInit(KernelContext*,
       static_cast<const CountOptions&>(*args.options));
 }
 
+// ----------------------------------------------------------------------
+// Distinct Count implementation
+
+template <typename Type>
+struct CountDistinctImpl : public ScalarAggregator {
+  using MemoTable = typename arrow::internal::HashTraits<Type>::MemoTableType;
+
+  explicit CountDistinctImpl(MemoryPool* memory_pool, CountOptions options)
+      : options(std::move(options)), memo_table_(new MemoTable(memory_pool, 
0)) {}
+
+  Status Consume(KernelContext*, const ExecBatch& batch) override {
+    if (batch[0].is_array()) {
+      const ArrayData& arr = *batch[0].array();
+      auto visit_null = [&]() {
+        if (this->nulls > 0) return Status::OK();
+        ++this->nulls;
+        return Status::OK();
+      };
+      auto visit_value = [&](typename Type::c_type arg) {
+        int y;
+        RETURN_NOT_OK(memo_table_->GetOrInsert(arg, &y));
+        return Status::OK();
+      };
+      RETURN_NOT_OK(VisitArrayDataInline<Type>(arr, visit_value, visit_null));
+      this->non_nulls += this->memo_table_->size();
+    } else {
+      const Scalar& input = *batch[0].scalar();
+      this->nulls += !input.is_valid * batch.length;
+      this->non_nulls += input.is_valid * batch.length;
+    }

Review comment:
       *all this being the `batch.length` usage in the aggregate kernels, which 
previously mishandled this case.




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