[GitHub] [arrow] liyafan82 commented on a change in pull request #7544: ARROW-7285: [C++] ensure C++ implementation meets clarified dictionary spec

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

liyafan82 commented on a change in pull request #7544:
URL: https://github.com/apache/arrow/pull/7544#discussion_r448166294



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File path: cpp/src/arrow/ipc/reader.cc
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@@ -684,7 +685,19 @@ Status ReadDictionary(const Buffer& metadata, DictionaryMemo* dictionary_memo,
     return Status::Invalid("Dictionary record batch must only contain one field");
   }
   auto dictionary = batch->column(0);
-  return dictionary_memo->AddDictionary(id, dictionary);
+  if (dictionary_batch->isDelta()) {
+    std::shared_ptr<Array> originalDict, combinedDict;
+    RETURN_NOT_OK(dictionary_memo->GetDictionary(id, &originalDict));
+    ArrayVector dictsToCombine{originalDict, dictionary};
+    ARROW_ASSIGN_OR_RAISE(combinedDict, Concatenate(dictsToCombine, options.memory_pool));

Review comment:
       Good suggestion. I have extracted it into the AddDirectoryDelta method. 




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