java/source/dataset.rst

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..   http://www.apache.org/licenses/LICENSE-2.0

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.. specific language governing permissions and limitations
.. under the License.

.. _arrow-dataset:

=======
Dataset
=======

* `Arrow Java Dataset`_: Java implementation of Arrow Datasets library. Implement Dataset Java API by JNI to C++.

.. contents::

Constructing Datasets
=====================

We can construct a dataset with an auto-inferred schema.

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import java.util.stream.StreamSupport;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   ScanOptions options = new ScanOptions(/*batchSize*/ 100);
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
       Dataset dataset = datasetFactory.finish();
       Scanner scanner = dataset.newScan(options)
   ) {
       System.out.println(StreamSupport.stream(scanner.scan().spliterator(), false).count());
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   1

Let construct our dataset with predefined schema.

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import java.util.stream.StreamSupport;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   ScanOptions options = new ScanOptions(/*batchSize*/ 100);
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
       Dataset dataset = datasetFactory.finish(datasetFactory.inspect());
       Scanner scanner = dataset.newScan(options)
   ) {
       System.out.println(StreamSupport.stream(scanner.scan().spliterator(), false).count());
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   1

Getting the Schema
==================

During Dataset Construction
***************************

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.types.pojo.Schema;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri)
   ) {
       Schema schema = datasetFactory.inspect();

       System.out.println(schema);
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   Schema<id: Int(32, true), name: Utf8>(metadata: {parquet.avro.schema={"type":"record","name":"User","namespace":"org.apache.arrow.dataset","fields":[{"name":"id","type":["int","null"]},{"name":"name","type":["string","null"]}]}, writer.model.name=avro})

From a Dataset
**************

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.types.pojo.Schema;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   ScanOptions options = new ScanOptions(/*batchSize*/ 1);
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
       Dataset dataset = datasetFactory.finish();
       Scanner scanner = dataset.newScan(options)
   ) {
       Schema schema = scanner.schema();

       System.out.println(schema);
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   Schema<id: Int(32, true), name: Utf8>(metadata: {parquet.avro.schema={"type":"record","name":"User","namespace":"org.apache.arrow.dataset","fields":[{"name":"id","type":["int","null"]},{"name":"name","type":["string","null"]}]}, writer.model.name=avro})

Query Parquet File
==================

Let query information for a parquet file.

Query Data Content For File
***************************

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.VectorSchemaRoot;
   import org.apache.arrow.vector.ipc.ArrowReader;

   import java.io.IOException;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   ScanOptions options = new ScanOptions(/*batchSize*/ 100);
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
       Dataset dataset = datasetFactory.finish();
       Scanner scanner = dataset.newScan(options)
   ) {
       scanner.scan().forEach(scanTask -> {
           try (ArrowReader reader = scanTask.execute()) {
               while (reader.loadNextBatch()) {
                   try (VectorSchemaRoot root = reader.getVectorSchemaRoot()) {
                       System.out.print(root.contentToTSVString());
                   }
               }
           } catch (IOException e) {
               e.printStackTrace();
           }
       });
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   id    name
   1    David
   2    Gladis
   3    Juan

Query Data Content For Directory
********************************

Consider that we have these files: data1: 3 rows, data2: 3 rows and data3: 250 rows.

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.VectorSchemaRoot;
   import org.apache.arrow.vector.ipc.ArrowReader;

   import java.io.IOException;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/";
   ScanOptions options = new ScanOptions(/*batchSize*/ 100);
   try (BufferAllocator allocator = new RootAllocator();
        DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
        Dataset dataset = datasetFactory.finish();
        Scanner scanner = dataset.newScan(options)
   ) {
       scanner.scan().forEach(scanTask-> {
           final int[] count = {1};
           try (ArrowReader reader = scanTask.execute()) {
               while (reader.loadNextBatch()) {
                   try (VectorSchemaRoot root = reader.getVectorSchemaRoot()) {
                       System.out.println("Batch: " + count[0]++ + ", RowCount: " + root.getRowCount());
                   }
               }
           } catch (IOException e) {
               e.printStackTrace();
           }
       });
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   Batch: 1, RowCount: 3
   Batch: 2, RowCount: 3
   Batch: 3, RowCount: 100
   Batch: 4, RowCount: 100
   Batch: 5, RowCount: 50

Query Data Content with Projection
**********************************

In case we need to project only certain columns we could configure ScanOptions with projections needed.

.. testcode::

   import org.apache.arrow.dataset.file.FileFormat;
   import org.apache.arrow.dataset.file.FileSystemDatasetFactory;
   import org.apache.arrow.dataset.jni.NativeMemoryPool;
   import org.apache.arrow.dataset.scanner.ScanOptions;
   import org.apache.arrow.dataset.scanner.Scanner;
   import org.apache.arrow.dataset.source.Dataset;
   import org.apache.arrow.dataset.source.DatasetFactory;
   import org.apache.arrow.memory.BufferAllocator;
   import org.apache.arrow.memory.RootAllocator;
   import org.apache.arrow.vector.VectorSchemaRoot;
   import org.apache.arrow.vector.ipc.ArrowReader;

   import java.io.IOException;

   String uri = "file:" + System.getProperty("user.dir") + "/thirdpartydeps/parquetfiles/data1.parquet";
   String[] projection = new String[] {"name"};
   ScanOptions options = new ScanOptions(/*batchSize*/ 100, Optional.of(projection));
   try (
       BufferAllocator allocator = new RootAllocator();
       DatasetFactory datasetFactory = new FileSystemDatasetFactory(allocator, NativeMemoryPool.getDefault(), FileFormat.PARQUET, uri);
       Dataset dataset = datasetFactory.finish();
       Scanner scanner = dataset.newScan(options)
   ) {
       scanner.scan().forEach(scanTask-> {
           try (ArrowReader reader = scanTask.execute()) {
               while (reader.loadNextBatch()) {
                   try (VectorSchemaRoot root = reader.getVectorSchemaRoot()) {
                       System.out.print(root.contentToTSVString());
                   }
               }
           } catch (IOException e) {
               e.printStackTrace();
           }
       });
   } catch (Exception e) {
       e.printStackTrace();
   }

.. testoutput::

   name
   David
   Gladis
   Juan


.. _Arrow Java Dataset: https://arrow.apache.org/docs/dev/java/dataset.html