Add hasNull statistic reading ability to ORC

[devavret]

Description

Adds the ability for ORC statistics reader to read the value ColumnStatistics::hasNull.

Contributes to #7087. Does not close it because the issue also requires the ability to write the field in the orc writer.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[devavret]

Note: The stats values for columns start from index 1. See similar test OrcStatisticsTest.Basic:

cudf/cpp/tests/io/orc_test.cpp

Lines 1015 to 1024 in 398aebe

	auto validate_statistics = [&](std::vector<cudf_io::column_statistics> const& stats) {
	auto& s0 = stats[0];
	EXPECT_EQ(*s0.number_of_values, 9ul);
	auto& s1 = stats[1];
	EXPECT_EQ(*s1.number_of_values, 4ul);
	auto& ts1 = std::get<cudf_io::integer_statistics>(s1.type_specific_stats);
	EXPECT_EQ(*ts1.minimum, 1);
	EXPECT_EQ(*ts1.maximum, 7);
	EXPECT_EQ(*ts1.sum, 16);

[codecov]

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Additional details and impacted files

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[ttnghia]

If this is going to be replaced by a round-trip write+read in the future then we may need to add comment to clarify that here.

[vuule]

Thank you for the patch!
Looks good, just one suggestion.

[vuule]

You can also validate stripe stats here
EXPECT_EQ(stats.stripes_stats[0][1].has_null, true);
EXPECT_EQ(stats.stripes_stats[0][2].has_null, false);

[devavret]

Can you give a little clarification on why the column stats start from index 1?

cudf/cpp/tests/io/orc_test.cpp

Lines 1015 to 1024 in 398aebe

	auto validate_statistics = [&](std::vector<cudf_io::column_statistics> const& stats) {
	auto& s0 = stats[0];
	EXPECT_EQ(*s0.number_of_values, 9ul);
	auto& s1 = stats[1];
	EXPECT_EQ(*s1.number_of_values, 4ul);
	auto& ts1 = std::get<cudf_io::integer_statistics>(s1.type_specific_stats);
	EXPECT_EQ(*ts1.minimum, 1);
	EXPECT_EQ(*ts1.maximum, 7);
	EXPECT_EQ(*ts1.sum, 16);

Is it because the first column is actually a Struct that contains these child columns?

[vuule]

Yes, ORC files have a root struct column.

[devavret]

@gpucibot merge