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| <article role="main" id="furo-main-content"> |
| <section id="tabular-data"> |
| <h1>Tabular Data<a class="headerlink" href="#tabular-data" title="Link to this heading">¶</a></h1> |
| <p>While arrays (aka: <a class="reference internal" href="vector.html"><span class="doc">ValueVector</span></a>) represent a one-dimensional sequence of |
| homogeneous values, data often comes in the form of two-dimensional sets of |
| heterogeneous data (such as database tables, CSV files…). Arrow provides |
| several abstractions to handle such data conveniently and efficiently.</p> |
| <section id="fields"> |
| <h2>Fields<a class="headerlink" href="#fields" title="Link to this heading">¶</a></h2> |
| <p>Fields are used to denote the particular columns of tabular data. |
| A field, i.e. an instance of <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/types/pojo/Field.html">Field</a>, holds together a field name, a data |
| type, and some optional key-value metadata.</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// Create a column "document" of string type with metadata</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.ArrowType</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.Field</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.FieldType</span><span class="p">;</span> |
| |
| <span class="n">Map</span><span class="o"><</span><span class="n">String</span><span class="p">,</span><span class="w"> </span><span class="n">String</span><span class="o">></span><span class="w"> </span><span class="n">metadata</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">HashMap</span><span class="o"><></span><span class="p">();</span> |
| <span class="n">metadata</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"A"</span><span class="p">,</span><span class="w"> </span><span class="s">"Id card"</span><span class="p">);</span> |
| <span class="n">metadata</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"B"</span><span class="p">,</span><span class="w"> </span><span class="s">"Passport"</span><span class="p">);</span> |
| <span class="n">metadata</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"C"</span><span class="p">,</span><span class="w"> </span><span class="s">"Visa"</span><span class="p">);</span> |
| <span class="n">Field</span><span class="w"> </span><span class="n">document</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">Field</span><span class="p">(</span><span class="s">"document"</span><span class="p">,</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">FieldType</span><span class="p">(</span><span class="kc">true</span><span class="p">,</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowType</span><span class="p">.</span><span class="na">Utf8</span><span class="p">(),</span><span class="w"> </span><span class="cm">/*dictionary*/</span><span class="w"> </span><span class="kc">null</span><span class="p">,</span><span class="w"> </span><span class="n">metadata</span><span class="p">),</span><span class="w"> </span><span class="cm">/*children*/</span><span class="w"> </span><span class="kc">null</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="schemas"> |
| <h2>Schemas<a class="headerlink" href="#schemas" title="Link to this heading">¶</a></h2> |
| <p>A <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/types/pojo/Schema.html">Schema</a> describes the overall structure consisting of any number of columns. It holds a sequence of fields together |
| with some optional schema-wide metadata (in addition to per-field metadata).</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// Create a schema describing datasets with two columns:</span> |
| <span class="c1">// a int32 column "A" and a utf8-encoded string column "B"</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.ArrowType</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.Field</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.FieldType</span><span class="p">;</span> |
| <span class="kn">import</span><span class="w"> </span><span class="nn">org.apache.arrow.vector.types.pojo.Schema</span><span class="p">;</span> |
| <span class="kn">import static</span><span class="w"> </span><span class="nn">java.util.Arrays.asList</span><span class="p">;</span> |
| |
| <span class="n">Map</span><span class="o"><</span><span class="n">String</span><span class="p">,</span><span class="w"> </span><span class="n">String</span><span class="o">></span><span class="w"> </span><span class="n">metadata</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">HashMap</span><span class="o"><></span><span class="p">();</span> |
| <span class="n">metadata</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"K1"</span><span class="p">,</span><span class="w"> </span><span class="s">"V1"</span><span class="p">);</span> |
| <span class="n">metadata</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="s">"K2"</span><span class="p">,</span><span class="w"> </span><span class="s">"V2"</span><span class="p">);</span> |
| <span class="n">Field</span><span class="w"> </span><span class="n">a</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">Field</span><span class="p">(</span><span class="s">"A"</span><span class="p">,</span><span class="w"> </span><span class="n">FieldType</span><span class="p">.</span><span class="na">nullable</span><span class="p">(</span><span class="k">new</span><span class="w"> </span><span class="n">ArrowType</span><span class="p">.</span><span class="na">Int</span><span class="p">(</span><span class="mi">32</span><span class="p">,</span><span class="w"> </span><span class="kc">true</span><span class="p">)),</span><span class="w"> </span><span class="kc">null</span><span class="p">);</span> |
| <span class="n">Field</span><span class="w"> </span><span class="n">b</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">Field</span><span class="p">(</span><span class="s">"B"</span><span class="p">,</span><span class="w"> </span><span class="n">FieldType</span><span class="p">.</span><span class="na">nullable</span><span class="p">(</span><span class="k">new</span><span class="w"> </span><span class="n">ArrowType</span><span class="p">.</span><span class="na">Utf8</span><span class="p">()),</span><span class="w"> </span><span class="kc">null</span><span class="p">);</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="k">new</span><span class="w"> </span><span class="n">Schema</span><span class="p">(</span><span class="n">asList</span><span class="p">(</span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">),</span><span class="w"> </span><span class="n">metadata</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="vectorschemaroot"> |
| <h2>VectorSchemaRoot<a class="headerlink" href="#vectorschemaroot" title="Link to this heading">¶</a></h2> |
| <p>A <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/VectorSchemaRoot.html">VectorSchemaRoot</a> is a container for batches of data. Batches flow through |
| VectorSchemaRoot as part of a pipeline.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>VectorSchemaRoot is somewhat analogous to tables or record batches in the |
| other Arrow implementations in that they all are 2D datasets, but their |
| usage is different.</p> |
| </div> |
| <p>The recommended usage is to create a single VectorSchemaRoot based on a known |
| schema and populate data over and over into that root in a stream of batches, |
| rather than creating a new instance each time (see <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/flight/package-summary.html">Flight</a> or |
| <code class="docutils literal notranslate"><span class="pre">ArrowFileWriter</span></code> as examples). Thus at any one point, a VectorSchemaRoot may |
| have data or may have no data (say it was transferred downstream or not yet |
| populated).</p> |
| <p>Here is an example of creating a VectorSchemaRoot:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">BitVector</span><span class="w"> </span><span class="n">bitVector</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">BitVector</span><span class="p">(</span><span class="s">"boolean"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">VarCharVector</span><span class="w"> </span><span class="n">varCharVector</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">VarCharVector</span><span class="p">(</span><span class="s">"varchar"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">bitVector</span><span class="p">.</span><span class="na">allocateNew</span><span class="p">();</span> |
| <span class="n">varCharVector</span><span class="p">.</span><span class="na">allocateNew</span><span class="p">();</span> |
| <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">bitVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">varCharVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="s">"test"</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">i</span><span class="p">).</span><span class="na">getBytes</span><span class="p">(</span><span class="n">StandardCharsets</span><span class="p">.</span><span class="na">UTF_8</span><span class="p">));</span> |
| <span class="p">}</span> |
| <span class="n">bitVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| <span class="n">varCharVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| |
| <span class="n">List</span><span class="o"><</span><span class="n">Field</span><span class="o">></span><span class="w"> </span><span class="n">fields</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">bitVector</span><span class="p">.</span><span class="na">getField</span><span class="p">(),</span><span class="w"> </span><span class="n">varCharVector</span><span class="p">.</span><span class="na">getField</span><span class="p">());</span> |
| <span class="n">List</span><span class="o"><</span><span class="n">FieldVector</span><span class="o">></span><span class="w"> </span><span class="n">vectors</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">bitVector</span><span class="p">,</span><span class="w"> </span><span class="n">varCharVector</span><span class="p">);</span> |
| <span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">vectorSchemaRoot</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">VectorSchemaRoot</span><span class="p">(</span><span class="n">fields</span><span class="p">,</span><span class="w"> </span><span class="n">vectors</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Data can be loaded into/unloaded from a VectorSchemaRoot via <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/VectorLoader.html">VectorLoader</a> |
| and <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/VectorUnloader.html">VectorUnloader</a>. They handle converting between VectorSchemaRoot and |
| <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/ipc/message/ArrowRecordBatch.html">ArrowRecordBatch</a> (a representation of a RecordBatch |
| <a class="reference external" href="https://arrow.apache.org/docs/format/Columnar.html#format-ipc" title="(in Apache Arrow v19.0.0)"><span class="xref std std-ref">IPC</span></a> message). For example:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// create a VectorSchemaRoot root1 and convert its data into recordBatch</span> |
| <span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">root1</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">VectorSchemaRoot</span><span class="p">(</span><span class="n">fields</span><span class="p">,</span><span class="w"> </span><span class="n">vectors</span><span class="p">);</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">root1</span><span class="p">);</span> |
| <span class="n">ArrowRecordBatch</span><span class="w"> </span><span class="n">recordBatch</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">unloader</span><span class="p">.</span><span class="na">getRecordBatch</span><span class="p">();</span> |
| |
| <span class="c1">// create a VectorSchemaRoot root2 and load the recordBatch</span> |
| <span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">root2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="p">.</span><span class="na">create</span><span class="p">(</span><span class="n">root1</span><span class="p">.</span><span class="na">getSchema</span><span class="p">(),</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">VectorLoader</span><span class="w"> </span><span class="n">loader</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">VectorLoader</span><span class="p">(</span><span class="n">root2</span><span class="p">);</span> |
| <span class="n">loader</span><span class="p">.</span><span class="na">load</span><span class="p">(</span><span class="n">recordBatch</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>A new VectorSchemaRoot can be sliced from an existing root without copying |
| data:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// 0 indicates start index (inclusive) and 5 indicated length (exclusive).</span> |
| <span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">newRoot</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">vectorSchemaRoot</span><span class="p">.</span><span class="na">slice</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="table"> |
| <h2>Table<a class="headerlink" href="#table" title="Link to this heading">¶</a></h2> |
| <p>A <a class="reference external" href="https://arrow.apache.org/docs/java/reference/org/apache/arrow/vector/table/Table.html">Table</a> is an immutable tabular data structure, very similar to VectorSchemaRoot, in that it is also built on ValueVectors and schemas. Unlike VectorSchemaRoot, Table is not designed for batch processing. Here is a version of the example above, showing how to create a Table, rather than a VectorSchemaRoot:</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">BitVector</span><span class="w"> </span><span class="n">bitVector</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">BitVector</span><span class="p">(</span><span class="s">"boolean"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">VarCharVector</span><span class="w"> </span><span class="n">varCharVector</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">VarCharVector</span><span class="p">(</span><span class="s">"varchar"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">bitVector</span><span class="p">.</span><span class="na">allocateNew</span><span class="p">();</span> |
| <span class="n">varCharVector</span><span class="p">.</span><span class="na">allocateNew</span><span class="p">();</span> |
| <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">bitVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">varCharVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="s">"test"</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">i</span><span class="p">).</span><span class="na">getBytes</span><span class="p">(</span><span class="n">StandardCharsets</span><span class="p">.</span><span class="na">UTF_8</span><span class="p">));</span> |
| <span class="p">}</span> |
| <span class="n">bitVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| <span class="n">varCharVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| |
| <span class="n">List</span><span class="o"><</span><span class="n">FieldVector</span><span class="o">></span><span class="w"> </span><span class="n">vectors</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">bitVector</span><span class="p">,</span><span class="w"> </span><span class="n">varCharVector</span><span class="p">);</span> |
| <span class="n">Table</span><span class="w"> </span><span class="n">table</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">Table</span><span class="p">(</span><span class="n">vectors</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>See the <a class="reference internal" href="table.html"><span class="doc">Table</span></a> documentation for more information.</p> |
| </section> |
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