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<div class="section" id="field-caches">
<h1>Field caches<a class="headerlink" href="#field-caches" title="Permalink to this headline">¶</a></h1>
<p>The default (<tt class="docutils literal"><span class="pre">filedb</span></tt>) backend uses <em>field caches</em> in certain circumstances.
The field cache basically pre-computes the order of documents in the index to
speed up sorting and faceting.</p>
<p>Generating field caches can take time the first time you sort/facet on a large
index. The field cache is kept in memory (and by default written to disk when it
is generated) so subsequent sorted/faceted searches should be faster.</p>
<p>The default caching policy never expires field caches, so reused searchers and/or
sorting a lot of different fields could use up quite a bit of memory with large
indexes.</p>
<div class="section" id="customizing-cache-behaviour">
<h2>Customizing cache behaviour<a class="headerlink" href="#customizing-cache-behaviour" title="Permalink to this headline">¶</a></h2>
<p>(The following API examples refer to the default <tt class="docutils literal"><span class="pre">filedb</span></tt> backend.)</p>
<p><em>By default</em>, Whoosh saves field caches to disk. To prevent a reader or searcher
from writing out field caches, do this before you start using it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">searcher</span><span class="o">.</span><span class="n">set_caching_policy</span><span class="p">(</span><span class="n">save</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
</pre></div>
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<p>By default, if caches are written to disk they are saved in the index directory.
To tell a reader or searcher to save cache files to a different location, create
a storage object and pass it to the <tt class="docutils literal"><span class="pre">storage</span></tt> keyword argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">whoosh.filedb.filestore</span> <span class="kn">import</span> <span class="n">FileStorage</span>
<span class="n">mystorage</span> <span class="o">=</span> <span class="n">FileStorage</span><span class="p">(</span><span class="s">"path/to/cachedir"</span><span class="p">)</span>
<span class="n">reader</span><span class="o">.</span><span class="n">set_caching_policy</span><span class="p">(</span><span class="n">storage</span><span class="o">=</span><span class="n">mystorage</span><span class="p">)</span>
</pre></div>
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<div class="section" id="creating-a-custom-caching-policy">
<h2>Creating a custom caching policy<a class="headerlink" href="#creating-a-custom-caching-policy" title="Permalink to this headline">¶</a></h2>
<p>Expert users who want to implement a custom caching policy (for example, to add
cache expiration) should subclass <tt class="xref py py-class docutils literal"><span class="pre">whoosh.filedb.fieldcache.FieldCachingPolicy</span></tt>.
Then you can pass an instance of your policy object to the <tt class="docutils literal"><span class="pre">set_caching_policy</span></tt>
method:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">searcher</span><span class="o">.</span><span class="n">set_caching_policy</span><span class="p">(</span><span class="n">MyPolicy</span><span class="p">())</span>
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<h3><a href="index.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">Field caches</a><ul>
<li><a class="reference internal" href="#customizing-cache-behaviour">Customizing cache behaviour</a></li>
<li><a class="reference internal" href="#creating-a-custom-caching-policy">Creating a custom caching policy</a></li>
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