Create an int8 column in read_csv when all elements are missing

cpp/src/io/csv/reader_impl.cu

@@ -538,8 +538,7 @@ void infer_column_types(parse_options const& parse_opts,
     auto const& stats = column_stats[inf_col_idx++];
     unsigned long long int_count_total =
       stats.big_int_count + stats.negative_small_int_count + stats.positive_small_int_count;
-
-    if (stats.null_count == num_records) {
+    if (stats.null_count == num_records or stats.total_count() == 0) {
       // Entire column is NULL; allocate the smallest amount of memory
       column_types[col_idx] = data_type(cudf::type_id::INT8);
     } else if (stats.string_count > 0L) {

cpp/src/io/utilities/column_type_histogram.hpp

@@ -33,6 +33,11 @@ struct column_type_histogram {
   cudf::size_type positive_small_int_count{};
   cudf::size_type big_int_count{};
   cudf::size_type bool_count{};
+  auto total_count() const
+  {
+    return null_count + float_count + datetime_count + string_count + negative_small_int_count +
+           positive_small_int_count + big_int_count + bool_count;
+  }
 };
 
 }  // namespace io

cpp/tests/io/csv_test.cpp

@@ -2244,6 +2244,27 @@ TEST_F(CsvReaderTest, CsvDefaultOptionsWriteReadMatch)
   EXPECT_EQ(new_table_and_metadata.metadata.column_names[1], "1");
 }
 
+TEST_F(CsvReaderTest, EmptyColumns)
+{
+  // First column only has empty fields. second column contains only "null" literals
+  std::string csv_in{",null\n,null"};
+
+  cudf::io::csv_reader_options in_opts =
+    cudf::io::csv_reader_options::builder(cudf::io::source_info{csv_in.c_str(), csv_in.size()})
+      .names({"a", "b", "c", "d"})
+      .header(-1);
+  // More elements in `names` than in the file; additional columns are filled with nulls
+  auto result = cudf::io::read_csv(in_opts);
+
+  const auto result_table = result.tbl->view();
+  ASSERT_EQ(result_table.num_columns(), 4);
+  // All columns should contain only nulls; expect INT8 type to use as little memory as possible
+  for (auto& column : result_table) {
+    ASSERT_EQ(column.type(), data_type{type_id::INT8});
+    ASSERT_EQ(column.null_count(), 2);
+  }
+}
+
 TEST_F(CsvReaderTest, BlankLineAfterFirstRow)
 {
   std::string csv_in{"12,9., 10\n\n"};