[GitHub] [arrow] sfc-gh-ebrossard commented on a diff in pull request #37016: GH-37014: [C++][Parquet] Preserve some Parquet distinct counts when merging stats

["sfc-gh-ebrossard (via GitHub)" <gi...@apache.org>]

sfc-gh-ebrossard commented on code in PR #37016:
URL: https://github.com/apache/arrow/pull/37016#discussion_r1283579685


##########
cpp/src/parquet/statistics_test.cc:
##########
@@ -645,6 +645,50 @@ class TestStatisticsHasFlag : public TestStatistics<TestType> {
                              EXPECT_FALSE(merged_statistics->HasDistinctCount());
                              EXPECT_FALSE(merged_statistics->Encode().has_distinct_count);
                            });
+
+    // Create a statistics object with zero distinct count. Merging preserves the distinct
+    // count if either side is zero.
+    std::shared_ptr<TypedStatistics<TestType>> statistics3;
+    std::shared_ptr<TypedStatistics<TestType>> statistics4;
+    {
+      EncodedStatistics encoded_statistics3;
+      encoded_statistics3.has_distinct_count = true;
+      encoded_statistics3.distinct_count = 0;
+      statistics3 = std::dynamic_pointer_cast<TypedStatistics<TestType>>(
+          Statistics::Make(this->schema_.Column(0), &encoded_statistics3,
+                           /*num_values=*/0));
+      EXPECT_TRUE(statistics3->HasDistinctCount());
+
+      EncodedStatistics encoded_statistics4;
+      encoded_statistics4.has_distinct_count = true;
+      encoded_statistics4.distinct_count = 10;
+      statistics4 = std::dynamic_pointer_cast<TypedStatistics<TestType>>(
+          Statistics::Make(this->schema_.Column(0), &encoded_statistics4,
+                           /*num_values=*/10));
+      EXPECT_TRUE(statistics4->HasDistinctCount());
+    }
+
+    // Both sides have 0 for the distinct count.
+    VerifyMergedStatistics(*statistics3, *statistics3,
+                           [](TypedStatistics<TestType>* merged_statistics) {
+                             EXPECT_TRUE(merged_statistics->HasDistinctCount());

Review Comment:
   Sorry, these tests are still failing. I'll see if I can get them running locally with https://arrow.apache.org/docs/developers/cpp/development.html#running-unit-tests and figure out what I've missed here.



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