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| <article role="main" id="furo-main-content"> |
| <section id="dataset"> |
| <h1>Dataset<a class="headerlink" href="#dataset" title="Link to this heading">¶</a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Experimental: The Java module <code class="docutils literal notranslate"><span class="pre">dataset</span></code> is currently under early |
| development. API might be changed in each release of Apache Arrow until it |
| gets mature.</p> |
| </div> |
| <p>Dataset is an universal layer in Apache Arrow for querying data in different |
| formats or in different partitioning strategies. Usually the data to be queried |
| is supposed to be located from a traditional file system, however Arrow Dataset |
| is not designed only for querying files but can be extended to serve all |
| possible data sources such as from inter-process communication or from other |
| network locations, etc.</p> |
| <section id="getting-started"> |
| <h2>Getting Started<a class="headerlink" href="#getting-started" title="Link to this heading">¶</a></h2> |
| <p>Currently supported file formats are:</p> |
| <ul class="simple"> |
| <li><p>Apache Arrow (<code class="docutils literal notranslate"><span class="pre">.arrow</span></code>)</p></li> |
| <li><p>Apache ORC (<code class="docutils literal notranslate"><span class="pre">.orc</span></code>)</p></li> |
| <li><p>Apache Parquet (<code class="docutils literal notranslate"><span class="pre">.parquet</span></code>)</p></li> |
| <li><p>Comma-Separated Values (<code class="docutils literal notranslate"><span class="pre">.csv</span></code>)</p></li> |
| <li><p>Line-delimited JSON Values (<code class="docutils literal notranslate"><span class="pre">.json</span></code>)</p></li> |
| </ul> |
| <p>Below shows a simplest example of using Dataset to query a Parquet file in Java:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// read data from file /opt/example.parquet</span> |
| <span class="n">String</span><span class="w"> </span><span class="n">uri</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"file:/opt/example.parquet"</span><span class="p">;</span> |
| <span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="cm">/*batchSize*/</span><span class="w"> </span><span class="mi">32768</span><span class="p">);</span> |
| <span class="k">try</span><span class="w"> </span><span class="p">(</span> |
| <span class="w"> </span><span class="n">BufferAllocator</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">RootAllocator</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">DatasetFactory</span><span class="w"> </span><span class="n">datasetFactory</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FileSystemDatasetFactory</span><span class="p">(</span> |
| <span class="w"> </span><span class="n">allocator</span><span class="p">,</span><span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">getDefault</span><span class="p">(),</span> |
| <span class="w"> </span><span class="n">FileFormat</span><span class="p">.</span><span class="na">PARQUET</span><span class="p">,</span><span class="w"> </span><span class="n">uri</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">Dataset</span><span class="w"> </span><span class="n">dataset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">datasetFactory</span><span class="p">.</span><span class="na">finish</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">Scanner</span><span class="w"> </span><span class="n">scanner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">dataset</span><span class="p">.</span><span class="na">newScan</span><span class="p">(</span><span class="n">options</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">ArrowReader</span><span class="w"> </span><span class="n">reader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">scanner</span><span class="p">.</span><span class="na">scanBatches</span><span class="p">()</span> |
| <span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">List</span><span class="o"><</span><span class="n">ArrowRecordBatch</span><span class="o">></span><span class="w"> </span><span class="n">batches</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrayList</span><span class="o"><></span><span class="p">();</span> |
| <span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">reader</span><span class="p">.</span><span class="na">loadNextBatch</span><span class="p">())</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">root</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getVectorSchemaRoot</span><span class="p">())</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="kd">final</span><span class="w"> </span><span class="n">VectorUnloader</span><span class="w"> </span><span class="n">unloader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VectorUnloader</span><span class="p">(</span><span class="n">root</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">batches</span><span class="p">.</span><span class="na">add</span><span class="p">(</span><span class="n">unloader</span><span class="p">.</span><span class="na">getRecordBatch</span><span class="p">());</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="c1">// do something with read record batches, for example:</span> |
| <span class="w"> </span><span class="n">analyzeArrowData</span><span class="p">(</span><span class="n">batches</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// finished the analysis of the data, close all resources:</span> |
| <span class="w"> </span><span class="n">AutoCloseables</span><span class="p">.</span><span class="na">close</span><span class="p">(</span><span class="n">batches</span><span class="p">);</span> |
| <span class="p">}</span><span class="w"> </span><span class="k">catch</span><span class="w"> </span><span class="p">(</span><span class="n">Exception</span><span class="w"> </span><span class="n">e</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">e</span><span class="p">.</span><span class="na">printStackTrace</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p><code class="docutils literal notranslate"><span class="pre">ArrowRecordBatch</span></code> is a low-level composite Arrow data exchange format |
| that doesn’t provide API to read typed data from it directly. |
| It’s recommended to use utilities <code class="docutils literal notranslate"><span class="pre">VectorLoader</span></code> to load it into a schema |
| aware container <code class="docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code> by which user could be able to access |
| decoded data conveniently in Java.</p> |
| <p>The <code class="docutils literal notranslate"><span class="pre">ScanOptions</span> <span class="pre">batchSize</span></code> argument takes effect only if it is set to a value |
| smaller than the number of rows in the recordbatch.</p> |
| </div> |
| <div class="admonition seealso"> |
| <p class="admonition-title">See also</p> |
| <p>Load record batches with <a class="reference internal" href="vector_schema_root.html"><span class="doc">VectorSchemaRoot</span></a>.</p> |
| </div> |
| </section> |
| <section id="schema"> |
| <h2>Schema<a class="headerlink" href="#schema" title="Link to this heading">¶</a></h2> |
| <p>Schema of the data to be queried can be inspected via method |
| <code class="docutils literal notranslate"><span class="pre">DatasetFactory#inspect()</span></code> before actually reading it. For example:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// read data from local file /opt/example.parquet</span> |
| <span class="n">String</span><span class="w"> </span><span class="n">uri</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"file:/opt/example.parquet"</span><span class="p">;</span> |
| <span class="n">BufferAllocator</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">RootAllocator</span><span class="p">(</span><span class="n">Long</span><span class="p">.</span><span class="na">MAX_VALUE</span><span class="p">);</span> |
| <span class="n">DatasetFactory</span><span class="w"> </span><span class="n">factory</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FileSystemDatasetFactory</span><span class="p">(</span><span class="n">allocator</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">getDefault</span><span class="p">(),</span><span class="w"> </span><span class="n">FileFormat</span><span class="p">.</span><span class="na">PARQUET</span><span class="p">,</span><span class="w"> </span><span class="n">uri</span><span class="p">);</span> |
| |
| <span class="c1">// inspect schema</span> |
| <span class="n">Schema</span><span class="w"> </span><span class="n">schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">factory</span><span class="p">.</span><span class="na">inspect</span><span class="p">();</span> |
| </pre></div> |
| </div> |
| <p>For some of the data format that is compatible with a user-defined schema, user |
| can use method <code class="docutils literal notranslate"><span class="pre">DatasetFactory#inspect(Schema</span> <span class="pre">schema)</span></code> to create the dataset:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">Schema</span><span class="w"> </span><span class="n">schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">createUserSchema</span><span class="p">()</span> |
| <span class="n">Dataset</span><span class="w"> </span><span class="n">dataset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">factory</span><span class="p">.</span><span class="na">finish</span><span class="p">(</span><span class="n">schema</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Otherwise when the non-parameter method <code class="docutils literal notranslate"><span class="pre">DatasetFactory#inspect()</span></code> is called, |
| schema will be inferred automatically from data source. The same as the result |
| of <code class="docutils literal notranslate"><span class="pre">DatasetFactory#inspect()</span></code>.</p> |
| <p>Also, if projector is specified during scanning (see next section |
| <a class="reference internal" href="#java-dataset-projection"><span class="std std-ref">Projection (Subset of Columns)</span></a>), the actual schema of output data can be got |
| within method <code class="docutils literal notranslate"><span class="pre">Scanner::schema()</span></code>:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">Scanner</span><span class="w"> </span><span class="n">scanner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">dataset</span><span class="p">.</span><span class="na">newScan</span><span class="p">(</span> |
| <span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="mi">32768</span><span class="p">,</span><span class="w"> </span><span class="n">Optional</span><span class="p">.</span><span class="na">of</span><span class="p">(</span><span class="k">new</span><span class="w"> </span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="p">{</span><span class="s">"id"</span><span class="p">,</span><span class="w"> </span><span class="s">"name"</span><span class="p">})));</span> |
| <span class="n">Schema</span><span class="w"> </span><span class="n">projectedSchema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">scanner</span><span class="p">.</span><span class="na">schema</span><span class="p">();</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="projection-subset-of-columns"> |
| <span id="java-dataset-projection"></span><h2>Projection (Subset of Columns)<a class="headerlink" href="#projection-subset-of-columns" title="Link to this heading">¶</a></h2> |
| <p>User can specify projections in ScanOptions. For example:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="n">projection</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">String</span><span class="o">[]</span><span class="w"> </span><span class="p">{</span><span class="s">"id"</span><span class="p">,</span><span class="w"> </span><span class="s">"name"</span><span class="p">};</span> |
| <span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="mi">32768</span><span class="p">,</span><span class="w"> </span><span class="n">Optional</span><span class="p">.</span><span class="na">of</span><span class="p">(</span><span class="n">projection</span><span class="p">));</span> |
| </pre></div> |
| </div> |
| <p>If no projection is needed, leave the optional projection argument absent in |
| ScanOptions:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="mi">32768</span><span class="p">,</span><span class="w"> </span><span class="n">Optional</span><span class="p">.</span><span class="na">empty</span><span class="p">());</span> |
| </pre></div> |
| </div> |
| <p>Or use shortcut constructor:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="mi">32768</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Then all columns will be emitted during scanning.</p> |
| </section> |
| <section id="projection-produce-new-columns-and-filters"> |
| <h2>Projection (Produce New Columns) and Filters<a class="headerlink" href="#projection-produce-new-columns-and-filters" title="Link to this heading">¶</a></h2> |
| <p>User can specify projections (new columns) or filters in ScanOptions using Substrait. For example:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">ByteBuffer</span><span class="w"> </span><span class="n">substraitExpressionFilter</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getSubstraitExpressionFilter</span><span class="p">();</span> |
| <span class="n">ByteBuffer</span><span class="w"> </span><span class="n">substraitExpressionProject</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getSubstraitExpressionProjection</span><span class="p">();</span> |
| <span class="c1">// Use Substrait APIs to create an Expression and serialize to a ByteBuffer</span> |
| <span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">.</span><span class="na">Builder</span><span class="p">(</span><span class="cm">/*batchSize*/</span><span class="w"> </span><span class="mi">32768</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">.</span><span class="na">columns</span><span class="p">(</span><span class="n">Optional</span><span class="p">.</span><span class="na">empty</span><span class="p">())</span> |
| <span class="w"> </span><span class="p">.</span><span class="na">substraitExpressionFilter</span><span class="p">(</span><span class="n">substraitExpressionFilter</span><span class="p">)</span> |
| <span class="w"> </span><span class="p">.</span><span class="na">substraitExpressionProjection</span><span class="p">(</span><span class="n">getSubstraitExpressionProjection</span><span class="p">())</span> |
| <span class="w"> </span><span class="p">.</span><span class="na">build</span><span class="p">();</span> |
| </pre></div> |
| </div> |
| <div class="admonition seealso"> |
| <p class="admonition-title">See also</p> |
| <dl class="simple"> |
| <dt><a class="reference internal" href="substrait.html"><span class="doc">Executing Projections and Filters Using Extended Expressions</span></a></dt><dd><p>Projections and Filters using Substrait.</p> |
| </dd> |
| </dl> |
| </div> |
| </section> |
| <section id="read-data-from-hdfs"> |
| <h2>Read Data from HDFS<a class="headerlink" href="#read-data-from-hdfs" title="Link to this heading">¶</a></h2> |
| <p><code class="docutils literal notranslate"><span class="pre">FileSystemDataset</span></code> supports reading data from non-local file systems. HDFS |
| support is included in the official Apache Arrow Java package releases and |
| can be used directly without re-building the source code.</p> |
| <p>To access HDFS data using Dataset API, pass a general HDFS URI to |
| <code class="docutils literal notranslate"><span class="pre">FilesSystemDatasetFactory</span></code>:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">String</span><span class="w"> </span><span class="n">uri</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"hdfs://{hdfs_host}:{port}/data/example.parquet"</span><span class="p">;</span> |
| <span class="n">BufferAllocator</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">RootAllocator</span><span class="p">(</span><span class="n">Long</span><span class="p">.</span><span class="na">MAX_VALUE</span><span class="p">);</span> |
| <span class="n">DatasetFactory</span><span class="w"> </span><span class="n">factory</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FileSystemDatasetFactory</span><span class="p">(</span><span class="n">allocator</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">getDefault</span><span class="p">(),</span><span class="w"> </span><span class="n">FileFormat</span><span class="p">.</span><span class="na">PARQUET</span><span class="p">,</span><span class="w"> </span><span class="n">uri</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="native-memory-management"> |
| <h2>Native Memory Management<a class="headerlink" href="#native-memory-management" title="Link to this heading">¶</a></h2> |
| <p>To gain better performance and reduce code complexity, Java |
| <code class="docutils literal notranslate"><span class="pre">FileSystemDataset</span></code> internally relies on C++ |
| <code class="docutils literal notranslate"><span class="pre">arrow::dataset::FileSystemDataset</span></code> via JNI. |
| As a result, all Arrow data read from <code class="docutils literal notranslate"><span class="pre">FileSystemDataset</span></code> is supposed to be |
| allocated off the JVM heap. To manage this part of memory, an utility class |
| <code class="docutils literal notranslate"><span class="pre">NativeMemoryPool</span></code> is provided to users.</p> |
| <p>As a basic example, by using a listenable <code class="docutils literal notranslate"><span class="pre">NativeMemoryPool</span></code>, user can pass |
| a listener hooking on C++ buffer allocation/deallocation:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">AtomicLong</span><span class="w"> </span><span class="n">reserved</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">AtomicLong</span><span class="p">(</span><span class="mi">0</span><span class="n">L</span><span class="p">);</span> |
| <span class="n">ReservationListener</span><span class="w"> </span><span class="n">listener</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ReservationListener</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="nd">@Override</span> |
| <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">reserve</span><span class="p">(</span><span class="kt">long</span><span class="w"> </span><span class="n">size</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">reserved</span><span class="p">.</span><span class="na">getAndAdd</span><span class="p">(</span><span class="n">size</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="nd">@Override</span> |
| <span class="w"> </span><span class="kd">public</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">unreserve</span><span class="p">(</span><span class="kt">long</span><span class="w"> </span><span class="n">size</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">reserved</span><span class="p">.</span><span class="na">getAndAdd</span><span class="p">(</span><span class="o">-</span><span class="n">size</span><span class="p">);</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">};</span> |
| <span class="n">NativeMemoryPool</span><span class="w"> </span><span class="n">pool</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">createListenable</span><span class="p">(</span><span class="n">listener</span><span class="p">);</span> |
| <span class="n">FileSystemDatasetFactory</span><span class="w"> </span><span class="n">factory</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FileSystemDatasetFactory</span><span class="p">(</span><span class="n">allocator</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">pool</span><span class="p">,</span><span class="w"> </span><span class="n">FileFormat</span><span class="p">.</span><span class="na">PARQUET</span><span class="p">,</span><span class="w"> </span><span class="n">uri</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Also, it’s a very common case to reserve the same amount of JVM direct memory |
| for the data read from datasets. For this use a built-in utility |
| class <code class="docutils literal notranslate"><span class="pre">DirectReservationListener</span></code> is provided:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">NativeMemoryPool</span><span class="w"> </span><span class="n">pool</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">createListenable</span><span class="p">(</span> |
| <span class="w"> </span><span class="n">DirectReservationListener</span><span class="p">.</span><span class="na">instance</span><span class="p">());</span> |
| </pre></div> |
| </div> |
| <p>This way, once the allocated byte count of Arrow buffers reaches the limit of |
| JVM direct memory, <code class="docutils literal notranslate"><span class="pre">OutOfMemoryError:</span> <span class="pre">Direct</span> <span class="pre">buffer</span> <span class="pre">memory</span></code> will |
| be thrown during scanning.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>The default instance <code class="docutils literal notranslate"><span class="pre">NativeMemoryPool.getDefaultMemoryPool()</span></code> does |
| nothing on buffer allocation/deallocation. It’s OK to use it in |
| the case of POC or testing, but for production use in complex environment, |
| it’s recommended to manage memory by using a listenable memory pool.</p> |
| </div> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>The <code class="docutils literal notranslate"><span class="pre">BufferAllocator</span></code> instance passed to <code class="docutils literal notranslate"><span class="pre">FileSystemDatasetFactory</span></code>’s |
| constructor is also aware of the overall memory usage of the produced |
| dataset instances. Once the Java buffers are created the passed allocator |
| will become their parent allocator.</p> |
| </div> |
| </section> |
| <section id="usage-notes"> |
| <h2>Usage Notes<a class="headerlink" href="#usage-notes" title="Link to this heading">¶</a></h2> |
| <section id="native-object-resource-management"> |
| <h3>Native Object Resource Management<a class="headerlink" href="#native-object-resource-management" title="Link to this heading">¶</a></h3> |
| <p>As another result of relying on JNI, all components related to |
| <code class="docutils literal notranslate"><span class="pre">FileSystemDataset</span></code> should be closed manually or use try-with-resources to |
| release the corresponding native objects after using. For example:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">String</span><span class="w"> </span><span class="n">uri</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"file:/opt/example.parquet"</span><span class="p">;</span> |
| <span class="n">ScanOptions</span><span class="w"> </span><span class="n">options</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ScanOptions</span><span class="p">(</span><span class="cm">/*batchSize*/</span><span class="w"> </span><span class="mi">32768</span><span class="p">);</span> |
| <span class="k">try</span><span class="w"> </span><span class="p">(</span> |
| <span class="w"> </span><span class="n">BufferAllocator</span><span class="w"> </span><span class="n">allocator</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">RootAllocator</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">DatasetFactory</span><span class="w"> </span><span class="n">factory</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FileSystemDatasetFactory</span><span class="p">(</span> |
| <span class="w"> </span><span class="n">allocator</span><span class="p">,</span><span class="w"> </span><span class="n">NativeMemoryPool</span><span class="p">.</span><span class="na">getDefault</span><span class="p">(),</span> |
| <span class="w"> </span><span class="n">FileFormat</span><span class="p">.</span><span class="na">PARQUET</span><span class="p">,</span><span class="w"> </span><span class="n">uri</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">Dataset</span><span class="w"> </span><span class="n">dataset</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">factory</span><span class="p">.</span><span class="na">finish</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">Scanner</span><span class="w"> </span><span class="n">scanner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">dataset</span><span class="p">.</span><span class="na">newScan</span><span class="p">(</span><span class="n">options</span><span class="p">)</span> |
| <span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| |
| <span class="w"> </span><span class="c1">// do something</span> |
| |
| <span class="p">}</span><span class="w"> </span><span class="k">catch</span><span class="w"> </span><span class="p">(</span><span class="n">Exception</span><span class="w"> </span><span class="n">e</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">e</span><span class="p">.</span><span class="na">printStackTrace</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>If user forgets to close them then native object leakage might be caused.</p> |
| </section> |
| <section id="batchsize"> |
| <h3>BatchSize<a class="headerlink" href="#batchsize" title="Link to this heading">¶</a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">batchSize</span></code> argument of <code class="docutils literal notranslate"><span class="pre">ScanOptions</span></code> is a limit on the size of an individual batch.</p> |
| <p>For example, let’s try to read a Parquet file with gzip compression and 3 row groups:</p> |
| <div class="highlight-default notranslate"><div class="highlight"><pre><span></span># Let configure ScanOptions as: |
| ScanOptions options = new ScanOptions(/*batchSize*/ 32768); |
| |
| $ parquet-tools meta data4_3rg_gzip.parquet |
| file schema: schema |
| age: OPTIONAL INT64 R:0 D:1 |
| name: OPTIONAL BINARY L:STRING R:0 D:1 |
| row group 1: RC:4 TS:182 OFFSET:4 |
| row group 2: RC:4 TS:190 OFFSET:420 |
| row group 3: RC:3 TS:179 OFFSET:838 |
| </pre></div> |
| </div> |
| <p>Here, we set the batchSize in ScanOptions to 32768. Because that’s greater |
| than the number of rows in the next batch, which is 4 rows because the first |
| row group has only 4 rows, then the program gets only 4 rows. The scanner |
| will not combine smaller batches to reach the limit, but it will split |
| large batches to stay under the limit. So in the case the row group had more |
| than 32768 rows, it would get split into blocks of 32768 rows or less.</p> |
| </section> |
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| <li><a class="reference internal" href="#">Dataset</a><ul> |
| <li><a class="reference internal" href="#getting-started">Getting Started</a></li> |
| <li><a class="reference internal" href="#schema">Schema</a></li> |
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