mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
173 lines
7.4 KiB
HTML
173 lines
7.4 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Hash access method specific configuration</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="am_conf.html" title="Chapter 2. Access Method Configuration" />
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<link rel="prev" href="bt_conf.html" title="Btree access method specific configuration" />
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<link rel="next" href="heap_conf.html" title="Heap access method specific configuration" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Hash access method specific
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configuration</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="bt_conf.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2. Access Method Configuration </th>
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<td width="20%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="hash_conf"></a>Hash access method specific
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configuration</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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There are a series of configuration tasks which you can
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perform when using the Hash access method. They are described
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in the following sections.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="am_conf_h_ffactor"></a>Page fill factor</h3>
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</div>
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</div>
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</div>
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<p>
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The density, or page fill factor, is an approximation of the
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number of keys allowed to accumulate in any one bucket,
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determining when the hash table grows or shrinks. If you know
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the average sizes of the keys and data in your data set,
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setting the fill factor can enhance performance. A reasonable
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rule to use to compute fill factor is:
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</p>
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<pre class="programlisting">(pagesize - 32) / (average_key_size + average_data_size + 8)</pre>
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<p>
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The desired density within the hash table can be specified
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by calling the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> method. If no density is
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specified, one will be selected dynamically as pages are
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filled.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="am_conf_h_hash"></a>Specifying a database hash</h3>
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</div>
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</div>
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</div>
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<p>
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The database hash determines in which bucket a particular
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key will reside. The goal of hashing keys is to distribute
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keys equally across the database pages, therefore it is
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important that the hash function work well with the specified
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keys so that the resulting bucket usage is relatively uniform.
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A hash function that does not work well can effectively turn
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into a sequential list.
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</p>
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<p>
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No hash performs equally well on all possible data sets. It
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is possible that applications may find that the default hash
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function performs poorly with a particular set of keys. The
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distribution resulting from the hash function can be checked
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using the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility. By comparing the number of hash buckets
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and the number of keys, one can decide if the entries are
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hashing in a well-distributed manner.
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</p>
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<p>
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The hash function for the hash table can be specified by
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calling the <a href="../api_reference/C/dbset_h_hash.html" class="olink">DB->set_h_hash()</a> method. If no hash function is
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specified, a default function will be used. Any
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application-specified hash function must take a reference to a
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<a href="../api_reference/C/db.html" class="olink">DB</a> object, a pointer to a byte string and its length, as
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arguments and return an unsigned, 32-bit hash value.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="am_conf_h_nelem"></a>Hash table size</h3>
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</div>
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</div>
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</div>
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<p>
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When setting up the hash database, knowing the expected
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number of elements that will be stored in the hash table is
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useful. This value can be used by the Hash access method
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implementation to more accurately construct the necessary
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number of buckets that the database will eventually
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require.
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</p>
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<p>
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The anticipated number of elements in the hash table can be
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specified by calling the <a href="../api_reference/C/dbset_h_nelem.html" class="olink">DB->set_h_nelem()</a> method. If not
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specified, or set too low, hash tables will expand gracefully
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as keys are entered, although a slight performance degradation
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may be noticed. In order for the estimated number of elements
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to be a useful value to Berkeley DB, the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a>
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method must also be called to set the page fill factor.
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</p>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="bt_conf.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="am_conf.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Btree access method specific configuration </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Heap access method specific
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configuration</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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