[GitHub] [arrow] Crystrix commented on a change in pull request #12032: ARROW-15126: [C++] Support Null type as group keys

[GitBox <gi...@apache.org>]

Crystrix commented on a change in pull request #12032:
URL: https://github.com/apache/arrow/pull/12032#discussion_r788387466



##########
File path: cpp/src/arrow/compute/kernels/hash_aggregate.cc
##########
@@ -147,10 +152,13 @@ struct GrouperImpl : Grouper {
       if (it_success.second) {
         // new key; update offsets and key_bytes
         ++num_groups_;
-        auto next_key_offset = static_cast<int32_t>(key_bytes_.size());
-        key_bytes_.resize(next_key_offset + key_length);
-        offsets_.push_back(next_key_offset + key_length);
-        memcpy(key_bytes_.data() + next_key_offset, key.c_str(), key_length);
+        if (key_length != 0) {

Review comment:
       Fixed

##########
File path: cpp/src/arrow/compute/kernels/row_encoder.h
##########
@@ -231,6 +231,26 @@ struct VarLengthKeyEncoder : KeyEncoder {
   std::shared_ptr<DataType> type_;
 };
 
+struct NullKeyEncoder : KeyEncoder {
+  void AddLength(const Datum&, int64_t batch_length, int32_t* lengths) override {
+    return;
+  }

Review comment:
       Fixed




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