R-JunmingChen commented on code in PR #37100:
URL: https://github.com/apache/arrow/pull/37100#discussion_r1364758859


##########
cpp/src/arrow/compute/kernels/aggregate_basic_internal.h:
##########
@@ -912,6 +912,122 @@ struct NullMinMaxImpl : public ScalarAggregator {
   }
 };
 
+template <SimdLevel::type SimdLevel>
+struct DictionaryMinMaxImpl : public ScalarAggregator {
+  using ThisType = DictionaryMinMaxImpl<SimdLevel>;
+
+  DictionaryMinMaxImpl(std::shared_ptr<DataType> out_type, 
ScalarAggregateOptions options)
+      : options(std::move(options)),
+        out_type(std::move(out_type)),
+        has_nulls(false),
+        count(0),
+        min(nullptr),
+        max(nullptr) {
+    this->options.min_count = std::max<uint32_t>(1, this->options.min_count);
+  }
+
+  Status Consume(KernelContext* ctx, const ExecSpan& batch) override {
+    if (batch[0].is_scalar()) {
+      return Status::NotImplemented("No min/max implemented for 
DictionaryScalar");
+    }
+
+    DictionaryArray dict_arr(batch[0].array.ToArrayData());
+    ARROW_ASSIGN_OR_RAISE(auto compacted_arr, 
dict_arr.Compact(ctx->memory_pool()));
+    const DictionaryArray& compacted_dict_arr =
+        checked_cast<const DictionaryArray&>(*compacted_arr);
+    if (compacted_dict_arr.length() - compacted_dict_arr.null_count() == 0) {
+      return Status::OK();
+    }
+    this->has_nulls |= compacted_dict_arr.null_count() > 0;
+    this->count += compacted_dict_arr.length() - 
compacted_dict_arr.null_count();

Review Comment:
   Sure. So it's a feature that `DictionaryArray::null_count()` only counts the 
number of nulls among the dict indices? Do we want `null_count()` return the 
same number as the dictionary array is decoded.



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