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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 15.7 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 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 68.5. Examples</td></tr></table></div></body></html>
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