pitrou commented on code in PR #37100:
URL: https://github.com/apache/arrow/pull/37100#discussion_r1299914912


##########
cpp/src/arrow/compute/kernels/aggregate_basic_internal.h:
##########
@@ -891,6 +891,105 @@ 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*, const ExecSpan& batch) override {
+    if (batch[0].is_scalar()) {
+      return Status::NotImplemented("No min/max implemented for 
DictionaryScalar");
+    }
+
+    DictionaryArray arr(batch[0].array.ToArrayData());
+    this->has_nulls = arr.null_count() > 0;
+    this->count += arr.length() - arr.null_count();
+
+    Datum dict_values(arr.dictionary());
+    ARROW_ASSIGN_OR_RAISE(Datum result, MinMax(std::move(dict_values)));

Review Comment:
   Hmm, perhaps a more efficient routine can be written specifically for this 
(find all dictionary indices actually used in a dictionary array)? Going 
through "is_in" and "filter" sounds quite expensive.



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