[GitHub] [arrow] wesm commented on a change in pull request #7033: ARROW-7759: [C++][Dataset] Add CsvFileFormat

[GitBox <gi...@apache.org>]

wesm commented on a change in pull request #7033:
URL: https://github.com/apache/arrow/pull/7033#discussion_r416152095



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File path: cpp/src/arrow/dataset/file_csv.cc
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@@ -0,0 +1,99 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/dataset/file_csv.h"
+
+#include <memory>
+#include <string>
+
+#include "arrow/csv/options.h"
+#include "arrow/csv/reader.h"
+#include "arrow/dataset/dataset_internal.h"
+#include "arrow/dataset/file_base.h"
+#include "arrow/dataset/type_fwd.h"
+#include "arrow/dataset/visibility.h"
+#include "arrow/result.h"
+#include "arrow/util/iterator.h"
+
+namespace arrow {
+namespace dataset {
+
+using internal::checked_cast;
+using internal::checked_pointer_cast;
+
+static inline csv::ReadOptions default_read_options() {
+  auto defaults = csv::ReadOptions::Defaults();
+  defaults.use_threads = false;
+  return defaults;
+}
+
+static inline Result<std::shared_ptr<csv::StreamingReader>> OpenReader(
+    const FileSource& source, const CsvFileFormat& format) {
+  ARROW_ASSIGN_OR_RAISE(auto input, source.Open());
+
+  auto maybe_reader = csv::StreamingReader::Make(
+      default_memory_pool(), std::move(input), default_read_options(),
+      format.parse_options, format.convert_options);
+  if (!maybe_reader.ok()) {
+    return maybe_reader.status().WithMessage("Could not open IPC input source '",
+                                             source.path(), "': ", maybe_reader.status());
+  }
+
+  return std::move(maybe_reader).ValueOrDie();
+}
+
+/// \brief A ScanTask backed by an Csv file.
+class CsvScanTask : public ScanTask {
+ public:
+  CsvScanTask(std::shared_ptr<const CsvFileFormat> format, FileSource source,
+              std::shared_ptr<ScanOptions> options, std::shared_ptr<ScanContext> context)
+      : ScanTask(std::move(options), std::move(context)),
+        format_(std::move(format)),
+        source_(std::move(source)) {}
+
+  Result<RecordBatchIterator> Execute() override {
+    ARROW_ASSIGN_OR_RAISE(auto reader, OpenReader(source_, *format_));

Review comment:
       In database systems, it is required to use a pre-determined schema when reading CSV files. If we want to support schema inference, we could use a part of a file to "sniff" the schema but then assume it going forward. That's a bit better than forcing people to write down the schema




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