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 can help create one."><meta property="og:image" content="https://arrow.apache.org/img/arrow-logo_horizontal_black-txt_white-bg.png"><meta property="og:image:alt" content="Apache Arrow logo, displaying the triple chevron image adjacent to the text"><meta name="twitter:card" content="summary_large_image"><meta name="twitter:creator" content="@apachearrow"><meta name="twitter:site" content="@apachearrow"><!-- mathjax --><script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js" integrity="sha256-nvJJv9wWKEm88qvoQl9ekL2J+k/RWIsaSScxxlsrv8k=" crossorigin="anonymous"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/config/TeX-AMS-MML_HTMLorMML.js" integrity="sha256-84DKXVJXs0/F8OTMzX4UR909+jtl4G7SPypPavF+GfA=" crossorigin="anonymous"></script><!--[if lt IE 9]>
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   <main id="main" class="col-md-9"><div class="page-header">
       <img src="" class="logo" alt=""><h1>Scan the contents of a dataset</h1>
       <small class="dont-index">Source: <a href="https://github.com/apache/arrow/blob/master/r/R/dataset-scan.R" class="external-link"><code>R/dataset-scan.R</code></a></small>
       <div class="d-none name"><code>Scanner.Rd</code></div>
     </div>

     <div class="ref-description section level2">
     <p>A <code>Scanner</code> iterates over a <a href="Dataset.html">Dataset</a>'s fragments and returns data
 according to given row filtering and column projection. A <code>ScannerBuilder</code>
 can help create one.</p>
     </div>


     <div class="section level2">
     <h2 id="factory">Factory<a class="anchor" aria-label="anchor" href="#factory"></a></h2>


 <p><code>Scanner$create()</code> wraps the <code>ScannerBuilder</code> interface to make a <code>Scanner</code>.
 It takes the following arguments:</p><ul><li><p><code>dataset</code>: A <code>Dataset</code> or <code>arrow_dplyr_query</code> object, as returned by the
 <code>dplyr</code> methods on <code>Dataset</code>.</p></li>
 <li><p><code>projection</code>: A character vector of column names to select columns or a
 named list of expressions</p></li>
 <li><p><code>filter</code>: A <code>Expression</code> to filter the scanned rows by, or <code>TRUE</code> (default)
 to keep all rows.</p></li>
 <li><p><code>use_threads</code>: logical: should scanning use multithreading? Default <code>TRUE</code></p></li>
 <li><p><code>...</code>: Additional arguments, currently ignored</p></li>
 </ul></div>
     <div class="section level2">
     <h2 id="methods">Methods<a class="anchor" aria-label="anchor" href="#methods"></a></h2>


 <p><code>ScannerBuilder</code> has the following methods:</p><ul><li><p><code>$Project(cols)</code>: Indicate that the scan should only return columns given
 by <code>cols</code>, a character vector of column names or a named list of <a href="Expression.html">Expression</a>.</p></li>
 <li><p><code>$Filter(expr)</code>: Filter rows by an <a href="Expression.html">Expression</a>.</p></li>
 <li><p><code>$UseThreads(threads)</code>: logical: should the scan use multithreading?
 The method's default input is <code>TRUE</code>, but you must call the method to enable
 multithreading because the scanner default is <code>FALSE</code>.</p></li>
 <li><p><code>$BatchSize(batch_size)</code>: integer: Maximum row count of scanned record
 batches, default is 32K. If scanned record batches are overflowing memory
 then this method can be called to reduce their size.</p></li>
 <li><p><code>$schema</code>: Active binding, returns the <a href="Schema.html">Schema</a> of the Dataset</p></li>
 <li><p><code>$Finish()</code>: Returns a <code>Scanner</code></p></li>
 </ul><p><code>Scanner</code> currently has a single method, <code>$ToTable()</code>, which evaluates the
 query and returns an Arrow <a href="Table.html">Table</a>.</p>
     </div>

     <div class="section level2">
     <h2 id="ref-examples">Examples<a class="anchor" aria-label="anchor" href="#ref-examples"></a></h2>
     <div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="co"># Set up directory for examples</span></span></span>
 <span class="r-in"><span><span class="va">tf</span> <span class="op">&lt;-</span> <span class="fu"><a href="https://rdrr.io/r/base/tempfile.html" class="external-link">tempfile</a></span><span class="op">(</span><span class="op">)</span></span></span>
 <span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/files2.html" class="external-link">dir.create</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span></span></span>
 <span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/on.exit.html" class="external-link">on.exit</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/unlink.html" class="external-link">unlink</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span><span class="op">)</span></span></span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="fu"><a href="write_dataset.html">write_dataset</a></span><span class="op">(</span><span class="va">mtcars</span>, <span class="va">tf</span>, partitioning<span class="op">=</span><span class="st">"cyl"</span><span class="op">)</span></span></span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="va">ds</span> <span class="op">&lt;-</span> <span class="fu"><a href="open_dataset.html">open_dataset</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span></span></span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="va">scan_builder</span> <span class="op">&lt;-</span> <span class="va">ds</span><span class="op">$</span><span class="fu">NewScan</span><span class="op">(</span><span class="op">)</span></span></span>
 <span class="r-in"><span><span class="va">scan_builder</span><span class="op">$</span><span class="fu">Filter</span><span class="op">(</span><span class="va">Expression</span><span class="op">$</span><span class="fu">field_ref</span><span class="op">(</span><span class="st">"hp"</span><span class="op">)</span> <span class="op">&gt;</span> <span class="fl">100</span><span class="op">)</span></span></span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> ScannerBuilder</span>
 <span class="r-in"><span><span class="va">scan_builder</span><span class="op">$</span><span class="fu">Project</span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/list.html" class="external-link">list</a></span><span class="op">(</span>hp_times_ten <span class="op">=</span> <span class="fl">10</span> <span class="op">*</span> <span class="va">Expression</span><span class="op">$</span><span class="fu">field_ref</span><span class="op">(</span><span class="st">"hp"</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span></span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> ScannerBuilder</span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="co"># Once configured, call $Finish()</span></span></span>
 <span class="r-in"><span><span class="va">scanner</span> <span class="op">&lt;-</span> <span class="va">scan_builder</span><span class="op">$</span><span class="fu">Finish</span><span class="op">(</span><span class="op">)</span></span></span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="co"># Can get results as a table</span></span></span>
 <span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/as.data.frame.html" class="external-link">as.data.frame</a></span><span class="op">(</span><span class="va">scanner</span><span class="op">$</span><span class="fu">ToTable</span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></span>
 <span class="r-out co"><span class="r-pr">#&gt;</span>    hp_times_ten</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 1          1130</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 2          1090</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 3          1100</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 4          1100</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 5          1100</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 6          1050</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 7          1230</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 8          1230</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 9          1750</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 10         1750</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 11         2450</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 12         1800</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 13         1800</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 14         1800</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 15         2050</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 16         2150</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 17         2300</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 18         1500</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 19         1500</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 20         2450</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 21         1750</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 22         2640</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> 23         3350</span>
 <span class="r-in"><span></span></span>
 <span class="r-in"><span><span class="co"># Or as a RecordBatchReader</span></span></span>
 <span class="r-in"><span><span class="va">scanner</span><span class="op">$</span><span class="fu">ToRecordBatchReader</span><span class="op">(</span><span class="op">)</span></span></span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> RecordBatchReader</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> hp_times_ten: double</span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> </span>
 <span class="r-out co"><span class="r-pr">#&gt;</span> See $metadata for additional Schema metadata</span>
 </code></pre></div>
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	<div class="row">
	<main id="main" class="col-md-9"><div class="page-header">
	<img src="" class="logo" alt=""><h1>Scan the contents of a dataset</h1>
	<small class="dont-index">Source: <a href="https://github.com/apache/arrow/blob/master/r/R/dataset-scan.R" class="external-link"><code>R/dataset-scan.R</code></a></small>
	<div class="d-none name"><code>Scanner.Rd</code></div>
	</div>

	<div class="ref-description section level2">
	<p>A <code>Scanner</code> iterates over a <a href="Dataset.html">Dataset</a>'s fragments and returns data
	according to given row filtering and column projection. A <code>ScannerBuilder</code>
	can help create one.</p>
	</div>


	<div class="section level2">
	<h2 id="factory">Factory<a class="anchor" aria-label="anchor" href="#factory"></a></h2>


	<p><code>Scanner$create()</code> wraps the <code>ScannerBuilder</code> interface to make a <code>Scanner</code>.
	It takes the following arguments:</p><ul><li><p><code>dataset</code>: A <code>Dataset</code> or <code>arrow_dplyr_query</code> object, as returned by the
	<code>dplyr</code> methods on <code>Dataset</code>.</p></li>
	<li><p><code>projection</code>: A character vector of column names to select columns or a
	named list of expressions</p></li>
	<li><p><code>filter</code>: A <code>Expression</code> to filter the scanned rows by, or <code>TRUE</code> (default)
	to keep all rows.</p></li>
	<li><p><code>use_threads</code>: logical: should scanning use multithreading? Default <code>TRUE</code></p></li>
	<li><p><code>...</code>: Additional arguments, currently ignored</p></li>
	</ul></div>
	<div class="section level2">
	<h2 id="methods">Methods<a class="anchor" aria-label="anchor" href="#methods"></a></h2>


	<p><code>ScannerBuilder</code> has the following methods:</p><ul><li><p><code>$Project(cols)</code>: Indicate that the scan should only return columns given
	by <code>cols</code>, a character vector of column names or a named list of <a href="Expression.html">Expression</a>.</p></li>
	<li><p><code>$Filter(expr)</code>: Filter rows by an <a href="Expression.html">Expression</a>.</p></li>
	<li><p><code>$UseThreads(threads)</code>: logical: should the scan use multithreading?
	The method's default input is <code>TRUE</code>, but you must call the method to enable
	multithreading because the scanner default is <code>FALSE</code>.</p></li>
	<li><p><code>$BatchSize(batch_size)</code>: integer: Maximum row count of scanned record
	batches, default is 32K. If scanned record batches are overflowing memory
	then this method can be called to reduce their size.</p></li>
	<li><p><code>$schema</code>: Active binding, returns the <a href="Schema.html">Schema</a> of the Dataset</p></li>
	<li><p><code>$Finish()</code>: Returns a <code>Scanner</code></p></li>
	</ul><p><code>Scanner</code> currently has a single method, <code>$ToTable()</code>, which evaluates the
	query and returns an Arrow <a href="Table.html">Table</a>.</p>
	</div>

	<div class="section level2">
	<h2 id="ref-examples">Examples<a class="anchor" aria-label="anchor" href="#ref-examples"></a></h2>
	<div class="sourceCode"><pre class="sourceCode r"><code><span class="r-in"><span><span class="co"># Set up directory for examples</span></span></span>
	<span class="r-in"><span><span class="va">tf</span> <span class="op"><-</span> <span class="fu"><a href="https://rdrr.io/r/base/tempfile.html" class="external-link">tempfile</a></span><span class="op">(</span><span class="op">)</span></span></span>
	<span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/files2.html" class="external-link">dir.create</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span></span></span>
	<span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/on.exit.html" class="external-link">on.exit</a></span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/unlink.html" class="external-link">unlink</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span><span class="op">)</span></span></span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="fu"><a href="write_dataset.html">write_dataset</a></span><span class="op">(</span><span class="va">mtcars</span>, <span class="va">tf</span>, partitioning<span class="op">=</span><span class="st">"cyl"</span><span class="op">)</span></span></span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="va">ds</span> <span class="op"><-</span> <span class="fu"><a href="open_dataset.html">open_dataset</a></span><span class="op">(</span><span class="va">tf</span><span class="op">)</span></span></span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="va">scan_builder</span> <span class="op"><-</span> <span class="va">ds</span><span class="op">$</span><span class="fu">NewScan</span><span class="op">(</span><span class="op">)</span></span></span>
	<span class="r-in"><span><span class="va">scan_builder</span><span class="op">$</span><span class="fu">Filter</span><span class="op">(</span><span class="va">Expression</span><span class="op">$</span><span class="fu">field_ref</span><span class="op">(</span><span class="st">"hp"</span><span class="op">)</span> <span class="op">></span> <span class="fl">100</span><span class="op">)</span></span></span>
	<span class="r-out co"><span class="r-pr">#></span> ScannerBuilder</span>
	<span class="r-in"><span><span class="va">scan_builder</span><span class="op">$</span><span class="fu">Project</span><span class="op">(</span><span class="fu"><a href="https://rdrr.io/r/base/list.html" class="external-link">list</a></span><span class="op">(</span>hp_times_ten <span class="op">=</span> <span class="fl">10</span> <span class="op">*</span> <span class="va">Expression</span><span class="op">$</span><span class="fu">field_ref</span><span class="op">(</span><span class="st">"hp"</span><span class="op">)</span><span class="op">)</span><span class="op">)</span></span></span>
	<span class="r-out co"><span class="r-pr">#></span> ScannerBuilder</span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="co"># Once configured, call $Finish()</span></span></span>
	<span class="r-in"><span><span class="va">scanner</span> <span class="op"><-</span> <span class="va">scan_builder</span><span class="op">$</span><span class="fu">Finish</span><span class="op">(</span><span class="op">)</span></span></span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="co"># Can get results as a table</span></span></span>
	<span class="r-in"><span><span class="fu"><a href="https://rdrr.io/r/base/as.data.frame.html" class="external-link">as.data.frame</a></span><span class="op">(</span><span class="va">scanner</span><span class="op">$</span><span class="fu">ToTable</span><span class="op">(</span><span class="op">)</span><span class="op">)</span></span></span>
	<span class="r-out co"><span class="r-pr">#></span> hp_times_ten</span>
	<span class="r-out co"><span class="r-pr">#></span> 1 1130</span>
	<span class="r-out co"><span class="r-pr">#></span> 2 1090</span>
	<span class="r-out co"><span class="r-pr">#></span> 3 1100</span>
	<span class="r-out co"><span class="r-pr">#></span> 4 1100</span>
	<span class="r-out co"><span class="r-pr">#></span> 5 1100</span>
	<span class="r-out co"><span class="r-pr">#></span> 6 1050</span>
	<span class="r-out co"><span class="r-pr">#></span> 7 1230</span>
	<span class="r-out co"><span class="r-pr">#></span> 8 1230</span>
	<span class="r-out co"><span class="r-pr">#></span> 9 1750</span>
	<span class="r-out co"><span class="r-pr">#></span> 10 1750</span>
	<span class="r-out co"><span class="r-pr">#></span> 11 2450</span>
	<span class="r-out co"><span class="r-pr">#></span> 12 1800</span>
	<span class="r-out co"><span class="r-pr">#></span> 13 1800</span>
	<span class="r-out co"><span class="r-pr">#></span> 14 1800</span>
	<span class="r-out co"><span class="r-pr">#></span> 15 2050</span>
	<span class="r-out co"><span class="r-pr">#></span> 16 2150</span>
	<span class="r-out co"><span class="r-pr">#></span> 17 2300</span>
	<span class="r-out co"><span class="r-pr">#></span> 18 1500</span>
	<span class="r-out co"><span class="r-pr">#></span> 19 1500</span>
	<span class="r-out co"><span class="r-pr">#></span> 20 2450</span>
	<span class="r-out co"><span class="r-pr">#></span> 21 1750</span>
	<span class="r-out co"><span class="r-pr">#></span> 22 2640</span>
	<span class="r-out co"><span class="r-pr">#></span> 23 3350</span>
	<span class="r-in"><span></span></span>
	<span class="r-in"><span><span class="co"># Or as a RecordBatchReader</span></span></span>
	<span class="r-in"><span><span class="va">scanner</span><span class="op">$</span><span class="fu">ToRecordBatchReader</span><span class="op">(</span><span class="op">)</span></span></span>
	<span class="r-out co"><span class="r-pr">#></span> RecordBatchReader</span>
	<span class="r-out co"><span class="r-pr">#></span> hp_times_ten: double</span>
	<span class="r-out co"><span class="r-pr">#></span> </span>
	<span class="r-out co"><span class="r-pr">#></span> See $metadata for additional Schema metadata</span>
	</code></pre></div>
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