[GitHub] [arrow] westonpace commented on a change in pull request #10729: ARROW-12513: [C++][Parquet] Parquet Writer always puts null_count=0 in Parquet statistics for dictionary-encoded array with nulls

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westonpace commented on a change in pull request #10729:
URL: https://github.com/apache/arrow/pull/10729#discussion_r670978233



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File path: cpp/src/parquet/statistics.cc
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@@ -567,6 +568,27 @@ class TypedStatisticsImpl : public TypedStatistics<DType> {
     SetMinMaxPair(comparator_->GetMinMax(values));
   }
 
+  void UpdateArrowDictionary(const ::arrow::Array& indices,
+                             const ::arrow::Array& dictionary) {
+    IncrementNullCount(indices.null_count());
+    IncrementNumValues(indices.length() - indices.null_count());
+
+    if (indices.null_count() == indices.length()) {
+      return;
+    }
+
+    ::arrow::compute::ExecContext ctx(pool_);
+    PARQUET_ASSIGN_OR_THROW(auto referenced_indices,
+                            ::arrow::compute::Unique(indices, &ctx));
+    PARQUET_ASSIGN_OR_THROW(

Review comment:
       I'll create a follow-up.  Another approach I considered was that we could compute the unique indices and then pass those down as a selection filter to the GetMinMax function.  My goal was minimizing change to the existing code.
   
   Or, since we have all these compute kernels now, just use ::arrow::compute::MinMax and pass the min & max into the statistics directly.




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