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<span id="development-cpp-compute"></span><h1>Developing Arrow C++ Compute<a class="headerlink" href="#developing-arrow-c-compute" title="Link to this heading">#</a></h1>
<p>This section provides information for developers of the Arrow C++ Compute module.</p>
<section id="row-table">
<h2>Row Table<a class="headerlink" href="#row-table" title="Link to this heading">#</a></h2>
<p>The row table in Arrow represents data stored in row-major format. This format
is particularly useful for scenarios involving random access to individual rows
and where all columns are frequently accessed together. It is especially
advantageous for hash-table keys and facilitates efficient operations such as
grouping and hash joins by optimizing memory access patterns and data locality.</p>
<section id="metadata">
<h3>Metadata<a class="headerlink" href="#metadata" title="Link to this heading">#</a></h3>
<p>A row table is defined by its metadata, <code class="docutils literal notranslate"><span class="pre">RowTableMetadata</span></code>, which includes
information about its schema, alignment, and derived properties.</p>
<p>The schema specifies the types and order of columns. Each row in the row table
contains the data for each column in that logical order (the physical order may
vary; see <a class="reference internal" href="#row-encoding"><span class="std std-ref">Row Encoding</span></a> for details).</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Columns of nested types or large binary types are <strong>not</strong> supported in the
row table.</p>
</div>
<p>One important property derived from the schema is whether the row table is
fixed-length or varying-length. A fixed-length row table contains only
fixed-length columns, while a varying-length row table includes at least one
varying-length column. This distinction determines how data is stored and
accessed in the row table.</p>
<p>Each row in the row table is aligned to <code class="docutils literal notranslate"><span class="pre">RowTableMetadata::row_alignment</span></code>
bytes. Fixed-length columns with non-power-of-2 lengths are also aligned to
<code class="docutils literal notranslate"><span class="pre">RowTableMetadata::row_alignment</span></code> bytes. Varying-length columns are aligned to
<code class="docutils literal notranslate"><span class="pre">RowTableMetadata::string_alignment</span></code> bytes.</p>
</section>
<section id="buffer-layout">
<h3>Buffer Layout<a class="headerlink" href="#buffer-layout" title="Link to this heading">#</a></h3>
<p>Similar to most Arrow <code class="docutils literal notranslate"><span class="pre">Array</span></code>s, the row table consists of three buffers:</p>
<ul class="simple">
<li><p><strong>Null Masks Buffer</strong>: Indicates null values for each column in each row.</p></li>
<li><p><strong>Fixed-length Buffer</strong>: Stores row data for fixed-length tables or offsets to
varying-length data for varying-length tables.</p></li>
<li><p><strong>Varying-length Buffer</strong> (Optional): Contains row data for varying-length
tables; unused for fixed-length tables.</p></li>
</ul>
</section>
<section id="row-format">
<h3>Row Format<a class="headerlink" href="#row-format" title="Link to this heading">#</a></h3>
<section id="null-masks">
<h4>Null Masks<a class="headerlink" href="#null-masks" title="Link to this heading">#</a></h4>
<p>For each row, a contiguous sequence of bits represents whether each column in
that row is null. Each bit corresponds to a specific column, with <code class="docutils literal notranslate"><span class="pre">1</span></code>
indicating the value is null and <code class="docutils literal notranslate"><span class="pre">0</span></code> indicating the value is valid. Note that
this is the opposite of how the validity bitmap works for <code class="docutils literal notranslate"><span class="pre">Array</span></code>s. The null
mask for a row occupies <code class="docutils literal notranslate"><span class="pre">RowTableMetadata::null_masks_bytes_per_row</span></code> bytes.</p>
</section>
<section id="fixed-length-row-data">
<h4>Fixed-length Row Data<a class="headerlink" href="#fixed-length-row-data" title="Link to this heading">#</a></h4>
<p>In a fixed-length row table, row data is directly stored in the fixed-length
buffer. All columns in each row are stored sequentially. Notably, a <code class="docutils literal notranslate"><span class="pre">boolean</span></code>
column is special because, in a normal Arrow <code class="docutils literal notranslate"><span class="pre">Array</span></code>, it is stored using 1
bit, whereas in a row table, it occupies 1 byte. The varying-length buffer is
not used in this case.</p>
<p>For example, a row table with the schema <code class="docutils literal notranslate"><span class="pre">(int32,</span> <span class="pre">boolean)</span></code> and rows
<code class="docutils literal notranslate"><span class="pre">[[7,</span> <span class="pre">false],</span> <span class="pre">[8,</span> <span class="pre">true],</span> <span class="pre">[9,</span> <span class="pre">false],</span> <span class="pre">...]</span></code> is stored in the fixed-length
buffer as follows:</p>
<div class="pst-scrollable-table-container"><table class="table">
<thead>
<tr class="row-odd"><th class="head"><p>Row 0</p></th>
<th class="head"><p>Row 1</p></th>
<th class="head"><p>Row 2</p></th>
<th class="head"><p>…</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">7</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">0</span> <span class="pre">(padding)</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">8</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">1</span> <span class="pre">(padding)</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">9</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">0</span> <span class="pre">(padding)</span></code></p></td>
<td><p>…</p></td>
</tr>
</tbody>
</table>
</div>
</section>
<section id="offsets-for-varying-length-row-data">
<h4>Offsets for Varying-length Row Data<a class="headerlink" href="#offsets-for-varying-length-row-data" title="Link to this heading">#</a></h4>
<p>In a varying-length row table, the fixed-length buffer contains offsets to the
varying-length row data, which is stored separately in the optional
varying-length buffer. The offsets are of type <code class="docutils literal notranslate"><span class="pre">RowTableMetadata::offset_type</span></code>
(fixed as <code class="docutils literal notranslate"><span class="pre">int64_t</span></code>) and indicate the starting position of the row data for
each row.</p>
</section>
<section id="varying-length-row-data">
<h4>Varying-length Row Data<a class="headerlink" href="#varying-length-row-data" title="Link to this heading">#</a></h4>
<p>In a varying-length row table, the varying-length buffer contains the actual row
data, stored contiguously. The offsets in the fixed-length buffer point to the
starting position of each row’s data.</p>
<section id="row-encoding">
<span id="id1"></span><h5>Row Encoding<a class="headerlink" href="#row-encoding" title="Link to this heading">#</a></h5>
<p>A varying-length row is encoded as follows:</p>
<ul class="simple">
<li><p>Fixed-length columns are stored first.</p></li>
<li><p>A sequence of offsets to each varying-length column follows. Each offset is
32-bit and indicates the <strong>end</strong> position within the row data of the
corresponding varying-length column.</p></li>
<li><p>Varying-length columns are stored last.</p></li>
</ul>
<p>For example, a row table with the schema <code class="docutils literal notranslate"><span class="pre">(int32,</span> <span class="pre">string,</span> <span class="pre">string,</span> <span class="pre">int32)</span></code> and
rows <code class="docutils literal notranslate"><span class="pre">[[7,</span> <span class="pre">'Alice',</span> <span class="pre">'x',</span> <span class="pre">0],</span> <span class="pre">[8,</span> <span class="pre">'Bob',</span> <span class="pre">'y',</span> <span class="pre">1],</span> <span class="pre">[9,</span> <span class="pre">'Charlotte',</span> <span class="pre">'z',</span> <span class="pre">2],</span> <span class="pre">...]</span></code>
is stored as follows (assuming 8-byte alignment for varying-length columns):</p>
<p>Fixed-length buffer (row offsets):</p>
<div class="pst-scrollable-table-container"><table class="table">
<thead>
<tr class="row-odd"><th class="head"><p>Row 0</p></th>
<th class="head"><p>Row 1</p></th>
<th class="head"><p>Row 2</p></th>
<th class="head"><p>Row 3</p></th>
<th class="head"><p>…</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">32</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">64</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">104</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p>…</p></td>
</tr>
</tbody>
</table>
</div>
<p>Varying-length buffer (row data):</p>
<div class="pst-scrollable-table-container"><table class="table">
<thead>
<tr class="row-odd"><th class="head"><p>Row</p></th>
<th class="head"><p>Fixed-length Cols</p></th>
<th class="head"><p>Varying-length Offsets</p></th>
<th class="head"><p>Varying-length Cols</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>0</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">7</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">21</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">25</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">Alice~~~x~~~~~~~</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>1</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">8</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">1</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">19</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">25</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">Bob~~~~~y~~~~~~~</span></code></p></td>
</tr>
<tr class="row-even"><td><p>2</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">9</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">2</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">25</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0,</span> <span class="pre">33</span> <span class="pre">0</span> <span class="pre">0</span> <span class="pre">0</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">Charlotte~~~~~~~z~~~~~~~</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>3</p></td>
<td><p>…</p></td>
<td><p>…</p></td>
<td><p>…</p></td>
</tr>
</tbody>
</table>
</div>
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