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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>68.4. Implementation</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="gist-extensibility.html" title="68.3. Extensibility" /><link rel="next" href="gist-examples.html" title="68.5. Examples" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">68.4. Implementation</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="gist-extensibility.html" title="68.3. Extensibility">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="gist.html" title="Chapter 68. GiST Indexes">Up</a></td><th width="60%" align="center">Chapter 68. GiST Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="gist-examples.html" title="68.5. Examples">Next</a></td></tr></table><hr /></div><div class="sect1" id="GIST-IMPLEMENTATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">68.4. Implementation</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="gist-implementation.html#GIST-BUFFERING-BUILD">68.4.1. GiST Index Build Methods</a></span></dt></dl></div><div class="sect2" id="GIST-BUFFERING-BUILD"><div class="titlepage"><div><div><h3 class="title">68.4.1. GiST Index Build Methods</h3></div></div></div><p>
+   The simplest way to build a GiST index is just to insert all the entries,
+   one by one.  This tends to be slow for large indexes, because if the
+   index tuples are scattered across the index and the index is large enough
+   to not fit in cache, a lot of random I/O will be
+   needed.  <span class="productname">PostgreSQL</span> supports two alternative
+   methods for initial build of a GiST index: <em class="firstterm">sorted</em>
+   and <em class="firstterm">buffered</em> modes.
+  </p><p>
+   The sorted method is only available if each of the opclasses used by the
+   index provides a <code class="function">sortsupport</code> function, as described
+   in <a class="xref" href="gist-extensibility.html" title="68.3. Extensibility">Section 68.3</a>.  If they do, this method is
+   usually the best, so it is used by default.
+  </p><p>
+   The buffered method works by not inserting tuples directly into the index
+   right away.  It can dramatically reduce the amount of random I/O needed
+   for non-ordered data sets.  For well-ordered data sets the benefit is
+   smaller or non-existent, because only a small number of pages receive new
+   tuples at a time, and those pages fit in cache even if the index as a
+   whole does not.
+  </p><p>
+   The buffered method needs to call the <code class="function">penalty</code>
+   function more often than the simple method does, which consumes some
+   extra CPU resources. Also, the buffers need temporary disk space, up to
+   the size of the resulting index. Buffering can also influence the quality
+   of the resulting index, in both positive and negative directions. That
+   influence depends on various factors, like the distribution of the input
+   data and the operator class implementation.
+  </p><p>
+   If sorting is not possible, then by default a GiST index build switches
+   to the buffering method when the index size reaches
+   <a class="xref" href="runtime-config-query.html#GUC-EFFECTIVE-CACHE-SIZE">effective_cache_size</a>.  Buffering can be manually
+   forced or prevented by the <code class="literal">buffering</code> parameter to the
+   CREATE INDEX command.  The default behavior is good for most cases, but
+   turning buffering off might speed up the build somewhat if the input data
+   is ordered.
+  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="gist-extensibility.html" title="68.3. Extensibility">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="gist.html" title="Chapter 68. GiST Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="gist-examples.html" title="68.5. Examples">Next</a></td></tr><tr><td width="40%" align="left" valign="top">68.3. Extensibility </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 68.5. Examples</td></tr></table></div></body></html>
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