[GitHub] [arrow] mapleFU commented on a diff in pull request #34054: GH-34053: [C++][Parquet] Write parquet page index

["mapleFU (via GitHub)" <gi...@apache.org>]

mapleFU commented on code in PR #34054:
URL: https://github.com/apache/arrow/pull/34054#discussion_r1118029417


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cpp/src/parquet/statistics.cc:
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@@ -494,6 +494,8 @@ class TypedStatisticsImpl : public TypedStatistics<DType> {
                       int64_t null_count, int64_t distinct_count, bool has_min_max,
                       bool has_null_count, bool has_distinct_count, MemoryPool* pool)
       : TypedStatisticsImpl(descr, pool) {
+    has_null_count_ = has_null_count;
+    has_distinct_count_ = has_distinct_count;

Review Comment:
   I meet the same problem here, I think the syntax of "has_xxx" is like that, for a writer:
   * Writer can assure that if has right null-count ( if it not has any bugs )
   * Currently I found that ndv is never collected. If a user collect ndv in page1, but not collect ndv in page 2, it should be abandon.
   
   For reader:
   * When deserialize, reader should assume that ndv and null_count can be unset ( but currently, it doesn't work like this)
   * Deserialized statistics can call merge, but if either `null_count` or `ndv` is unset, all null_count should be discarded.



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