Fix column selection read_parquet benchmarks (#13082)

Helper function `get_col_names` in the Parquet reader benchmarks throws with nested columns. It should instead just ignore the children columns and return the top-level colum names.
Also renamed the function to better reflect what it does.

Authors:
  - Vukasin Milovanovic (https://github.com/vuule)

Approvers:
  - https://github.com/nvdbaranec
  - Yunsong Wang (https://github.com/PointKernel)

URL: #13082

cpp/benchmarks/io/orc/orc_reader_options.cpp

@@ -33,7 +33,7 @@ constexpr int64_t data_size = 512 << 20;
 // Each call reads roughly equal amounts of data
 constexpr int32_t chunked_read_num_chunks = 8;
 
-std::vector<std::string> get_col_names(cudf::io::source_info const& source)
+std::vector<std::string> get_top_level_col_names(cudf::io::source_info const& source)
 {
   auto const top_lvl_cols = cudf::io::read_orc_metadata(source).schema().root().children();
   std::vector<std::string> col_names;
@@ -79,7 +79,7 @@ void BM_orc_read_varying_options(nvbench::state& state,
   cudf::io::write_orc(options);
 
   auto const cols_to_read =
-    select_column_names(get_col_names(source_sink.make_source_info()), ColSelection);
+    select_column_names(get_top_level_col_names(source_sink.make_source_info()), ColSelection);
   cudf::io::orc_reader_options read_options =
     cudf::io::orc_reader_options::builder(source_sink.make_source_info())
       .columns(cols_to_read)

cpp/benchmarks/io/parquet/parquet_reader_options.cpp

@@ -30,7 +30,7 @@
 constexpr std::size_t data_size      = 512 << 20;
 constexpr std::size_t row_group_size = 128 << 20;
 
-std::vector<std::string> get_col_names(cudf::io::source_info const& source)
+std::vector<std::string> get_top_level_col_names(cudf::io::source_info const& source)
 {
   cudf::io::parquet_reader_options const read_options =
     cudf::io::parquet_reader_options::builder(source);
@@ -39,7 +39,6 @@ std::vector<std::string> get_col_names(cudf::io::source_info const& source)
   std::vector<std::string> names;
   names.reserve(schema.size());
   std::transform(schema.cbegin(), schema.cend(), std::back_inserter(names), [](auto const& c) {
-    CUDF_EXPECTS(c.children.empty(), "nested types are not supported");
     return c.name;
   });
   return names;
@@ -81,7 +80,7 @@ void BM_parquet_read_options(nvbench::state& state,
   cudf::io::write_parquet(options);
 
   auto const cols_to_read =
-    select_column_names(get_col_names(source_sink.make_source_info()), ColSelection);
+    select_column_names(get_top_level_col_names(source_sink.make_source_info()), ColSelection);
   cudf::io::parquet_reader_options read_options =
     cudf::io::parquet_reader_options::builder(source_sink.make_source_info())
       .columns(cols_to_read)