From 8632ca0da31b49fd318d8cc5972c7e65f1b7ced3 Mon Sep 17 00:00:00 2001
From: "Ram (Ramakrishna Prabhu)"
 <42624703+rgsl888prabhu@users.noreply.github.com>
Date: Mon, 22 Mar 2021 23:30:26 +0530
Subject: [PATCH] Fix ORC reader for empty DataFrame/Table (#7624)

`ff.types` by default will have a [main type as struct](https://github.com/rapidsai/cudf/blob/0146f743987a6f2a51aab08f34771eb4d3531afc/cpp/src/io/orc/writer_impl.cu#L1278) under which all other columns will originate. So, we need to skip first which is not a column and start with 1st index.
(Look for `Type Information` in [ORC Specification](https://orc.apache.org/specification/ORCv1/))
 Along with that, we should also take care of the scenario where user would specify specific column name to retrieve, but it doesn't exist in case of empty data frame/table.

Added test case to validate both scenario.

closes #7356

Authors:
  - Ram (Ramakrishna Prabhu) (@rgsl888prabhu)

Approvers:
  - Devavret Makkar (@devavret)
  - @nvdbaranec
  - Vukasin Milovanovic (@vuule)
  - Michael Wang (@isVoid)

URL: https://github.com/rapidsai/cudf/pull/7624
---
 cpp/src/io/orc/orc.cpp             |  6 ++++--
 cpp/src/io/orc/reader_impl.cu      |  3 +++
 python/cudf/cudf/tests/test_orc.py | 16 ++++++++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/cpp/src/io/orc/orc.cpp b/cpp/src/io/orc/orc.cpp
index 6c0216a6d6b..e1b6c3ace6c 100644
--- a/cpp/src/io/orc/orc.cpp
+++ b/cpp/src/io/orc/orc.cpp
@@ -538,26 +538,28 @@ std::vector<int> metadata::select_columns(std::vector<std::string> use_names,
   if (not use_names.empty()) {
     int index = 0;
     for (const auto &use_name : use_names) {
+      bool name_found = false;
       for (int i = 0; i < get_num_columns(); ++i, ++index) {
         if (index >= get_num_columns()) { index = 0; }
         if (get_column_name(index) == use_name) {
+          name_found = true;
           selection.emplace_back(index);
           if (ff.types[index].kind == orc::TIMESTAMP) { has_timestamp_column = true; }
           index++;
           break;
         }
       }
+      CUDF_EXPECTS(name_found, "Unknown column name : " + std::string(use_name));
     }
   } else {
     // For now, only select all leaf nodes
-    for (int i = 0; i < get_num_columns(); ++i) {
+    for (int i = 1; i < get_num_columns(); ++i) {
       if (ff.types[i].subtypes.empty()) {
         selection.emplace_back(i);
         if (ff.types[i].kind == orc::TIMESTAMP) { has_timestamp_column = true; }
       }
     }
   }
-  CUDF_EXPECTS(selection.size() > 0, "Filtered out all columns");
 
   return selection;
 }
diff --git a/cpp/src/io/orc/reader_impl.cu b/cpp/src/io/orc/reader_impl.cu
index e6e3ec69e43..9f88c6584ce 100644
--- a/cpp/src/io/orc/reader_impl.cu
+++ b/cpp/src/io/orc/reader_impl.cu
@@ -419,6 +419,9 @@ table_with_metadata reader::impl::read(size_type skip_rows,
   std::vector<std::unique_ptr<column>> out_columns;
   table_metadata out_metadata;
 
+  // There are no columns in table
+  if (_selected_columns.size() == 0) return {std::make_unique<table>(), std::move(out_metadata)};
+
   // Select only stripes required (aka row groups)
   const auto selected_stripes = _metadata->select_stripes(stripes, skip_rows, num_rows);
 
diff --git a/python/cudf/cudf/tests/test_orc.py b/python/cudf/cudf/tests/test_orc.py
index fa14a0a9690..faad489da86 100644
--- a/python/cudf/cudf/tests/test_orc.py
+++ b/python/cudf/cudf/tests/test_orc.py
@@ -740,6 +740,22 @@ def test_nanoseconds_overflow():
     assert_eq(expected.to_pandas(), pyarrow_got.to_pandas())
 
 
+def test_empty_dataframe():
+    buffer = BytesIO()
+    expected = cudf.DataFrame()
+    expected.to_orc(buffer)
+
+    # Raise error if column name is mentioned, but it doesn't exist.
+    with pytest.raises(RuntimeError):
+        cudf.read_orc(buffer, columns=["a"])
+
+    got_df = cudf.read_orc(buffer)
+    expected_pdf = pd.read_orc(buffer)
+
+    assert_eq(expected, got_df)
+    assert_eq(expected_pdf, got_df)
+
+
 @pytest.mark.parametrize(
     "data", [[None, ""], ["", None], [None, None], ["", ""]]
 )