docs/source/cpp/orc.rst

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.. default-domain:: cpp
.. highlight:: cpp

.. cpp:namespace:: arrow::adapters::orc

=============================
Reading and Writing ORC files
=============================

The `Apache ORC <http://orc.apache.org/>`_ project provides a
standardized open-source columnar storage format for use in data analysis
systems. It was created originally for use in `Apache Hadoop
<http://hadoop.apache.org/>`_ with systems like `Apache Drill
<http://drill.apache.org>`_, `Apache Hive <http://hive.apache.org>`_, `Apache
Impala <http://impala.apache.org>`_, and `Apache Spark
<http://spark.apache.org>`_ adopting it as a shared standard for high
performance data IO.

Apache Arrow is an ideal in-memory representation layer for data that is being read
or written with ORC files.

.. seealso::
   :ref:`ORC reader/writer API reference <cpp-api-orc>`.

Supported ORC features
==========================

The ORC format has many features, and we support a subset of them.

Data types
----------
Here are a list of ORC types and mapped Arrow types.

+-------------------+-----------------------------------+-----------+
| Logical type      | Mapped Arrow type                 | Notes     |
+===================+===================================+===========+
| BOOLEAN           | Boolean                           |           |
+-------------------+-----------------------------------+-----------+
| BYTE              | Int8                              |           |
+-------------------+-----------------------------------+-----------+
| SHORT             | Int16                             |           |
+-------------------+-----------------------------------+-----------+
| INT               | Int32                             |           |
+-------------------+-----------------------------------+-----------+
| LONG              | Int64                             |           |
+-------------------+-----------------------------------+-----------+
| FLOAT             | Float32                           |           |
+-------------------+-----------------------------------+-----------+
| DOUBLE            | Float64                           |           |
+-------------------+-----------------------------------+-----------+
| STRING            | String/LargeString                | \(1)      |
+-------------------+-----------------------------------+-----------+
| BINARY            | Binary/LargeBinary/FixedSizeBinary| \(1)      |
+-------------------+-----------------------------------+-----------+
| TIMESTAMP         | Timestamp/Date64                  | \(1) \(2) |
+-------------------+-----------------------------------+-----------+
| TIMESTAMP_INSTANT | Timestamp                         | \(2)      |
+-------------------+-----------------------------------+-----------+
| LIST              | List/LargeList/FixedSizeList      | \(1)      |
+-------------------+-----------------------------------+-----------+
| MAP               | Map                               |           |
+-------------------+-----------------------------------+-----------+
| STRUCT            | Struct                            |           |
+-------------------+-----------------------------------+-----------+
| UNION             | SparseUnion/DenseUnion            | \(1)      |
+-------------------+-----------------------------------+-----------+
| DECIMAL           | Decimal128/Decimal256             | \(1)      |
+-------------------+-----------------------------------+-----------+
| DATE              | Date32                            |           |
+-------------------+-----------------------------------+-----------+
| VARCHAR           | String                            | \(3)      |
+-------------------+-----------------------------------+-----------+
| CHAR              | String                            | \(3)      |
+-------------------+-----------------------------------+-----------+

* \(1) On the read side the ORC type is read as the first corresponding Arrow type in the table.

* \(2) On the write side the ORC TIMESTAMP_INSTANT is used when timezone is provided, otherwise
  ORC TIMESTAMP is used. On the read side both ORC TIMESTAMP and TIMESTAMP_INSTANT types are read
  as the Arrow Timestamp type with :cpp:enumerator:`arrow::TimeUnit::NANO` and timezone is set to
  UTC for ORC TIMESTAMP_INSTANT type only.

* \(3) On the read side both ORC CHAR and VARCHAR types are read as the Arrow String type. ORC CHAR
  and VARCHAR types are not supported on the write side.

Compression
-----------

+-------------------+
| Compression codec |
+===================+
| SNAPPY            |
+-------------------+
| GZIP/ZLIB         |
+-------------------+
| LZ4               |
+-------------------+
| ZSTD              |
+-------------------+

*Unsupported compression codec:* LZO.


Reading ORC Files
=================

The :class:`ORCFileReader` class reads data for an entire
file or stripe into an :class:`::arrow::Table`.

ORCFileReader
-------------

The :class:`ORCFileReader` class requires a
:class:`::arrow::io::RandomAccessFile` instance representing the input
file.

.. code-block:: cpp

    #include <arrow/adapters/orc/adapter.h>

    {
        // ...
        arrow::Status st;
        arrow::MemoryPool* pool = default_memory_pool();
        std::shared_ptr<arrow::io::RandomAccessFile> input = ...;

        // Open ORC file reader
        auto maybe_reader = arrow::adapters::orc::ORCFileReader::Open(input, pool);
        if (!maybe_reader.ok()) {
            // Handle error instantiating file reader...
        }
        std::unique_ptr<arrow::adapters::orc::ORCFileReader> reader = maybe_reader.ValueOrDie();

        // Read entire file as a single Arrow table
        auto maybe_table = reader->Read();
        if (!maybe_table.ok()) {
            // Handle error reading ORC data...
        }
        std::shared_ptr<arrow::Table> table = maybe_table.ValueOrDie();
    }


Writing ORC Files
=================

ORCFileWriter
-------------

An ORC file is written to a :class:`~arrow::io::OutputStream`.

.. code-block:: cpp

    #include <arrow/adapters/orc/adapter.h>
    {
        // Oneshot write
        // ...
        std::shared_ptr<arrow::io::OutputStream> output = ...;
        auto writer_options = WriterOptions();
        auto maybe_writer = arrow::adapters::orc::ORCFileWriter::Open(output.get(), writer_options);
        if (!maybe_writer.ok()) {
           // Handle error instantiating file writer...
        }
        std::unique_ptr<arrow::adapters::orc::ORCFileWriter> writer = maybe_writer.ValueOrDie();
        if (!(writer->Write(*input_table)).ok()) {
            // Handle write error...
        }
        if (!(writer->Close()).ok()) {
            // Handle close error...
        }
    }