mirror of
https://github.com/berkeleydb/libdb.git
synced 2024-11-16 17:16:25 +00:00
148 lines
7 KiB
HTML
148 lines
7 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!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>Hash access method specific configuration</title>
|
||
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
|
||
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
|
||
<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
|
||
<link rel="up" href="am_conf.html" title="Chapter 2. Access Method Configuration" />
|
||
<link rel="prev" href="bt_conf.html" title="Btree access method specific configuration" />
|
||
<link rel="next" href="heap_conf.html" title="Heap access method specific configuration" />
|
||
</head>
|
||
<body>
|
||
<div xmlns="" class="navheader">
|
||
<div class="libver">
|
||
<p>Library Version 11.2.5.2</p>
|
||
</div>
|
||
<table width="100%" summary="Navigation header">
|
||
<tr>
|
||
<th colspan="3" align="center">Hash access method specific configuration</th>
|
||
</tr>
|
||
<tr>
|
||
<td width="20%" align="left"><a accesskey="p" href="bt_conf.html">Prev</a> </td>
|
||
<th width="60%" align="center">Chapter 2.
|
||
Access Method Configuration
|
||
</th>
|
||
<td width="20%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
|
||
</tr>
|
||
</table>
|
||
<hr />
|
||
</div>
|
||
<div class="sect1" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h2 class="title" style="clear: both"><a id="hash_conf"></a>Hash access method specific configuration</h2>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="toc">
|
||
<dl>
|
||
<dt>
|
||
<span class="sect2">
|
||
<a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
|
||
</span>
|
||
</dt>
|
||
<dt>
|
||
<span class="sect2">
|
||
<a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
|
||
</span>
|
||
</dt>
|
||
<dt>
|
||
<span class="sect2">
|
||
<a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
|
||
</span>
|
||
</dt>
|
||
</dl>
|
||
</div>
|
||
<p>
|
||
There are a series of configuration tasks which you can perform when
|
||
using the Hash access method. They are described in the following sections.
|
||
</p>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="am_conf_h_ffactor"></a>Page fill factor</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>The density, or page fill factor, is an approximation of the number of
|
||
keys allowed to accumulate in any one bucket, determining when the hash
|
||
table grows or shrinks. If you know the average sizes of the keys and
|
||
data in your data set, setting the fill factor can enhance performance.
|
||
A reasonable rule to use to compute fill factor is:</p>
|
||
<pre class="programlisting">(pagesize - 32) / (average_key_size + average_data_size + 8)</pre>
|
||
<p>The desired density within the hash table can be specified by calling
|
||
the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> method. If no density is specified, one will
|
||
be selected dynamically as pages are filled.</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="am_conf_h_hash"></a>Specifying a database hash</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>The database hash determines in which bucket a particular key will reside.
|
||
The goal of hashing keys is to distribute keys equally across the database
|
||
pages, therefore it is important that the hash function work well with
|
||
the specified keys so that the resulting bucket usage is relatively
|
||
uniform. A hash function that does not work well can effectively turn
|
||
into a sequential list.</p>
|
||
<p>No hash performs equally well on all possible data sets. It is possible
|
||
that applications may find that the default hash function performs poorly
|
||
with a particular set of keys. The distribution resulting from the hash
|
||
function can be checked using the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility. By comparing the
|
||
number of hash buckets and the number of keys, one can decide if the entries
|
||
are hashing in a well-distributed manner.</p>
|
||
<p>The hash function for the hash table can be specified by calling the
|
||
<a href="../api_reference/C/dbset_h_hash.html" class="olink">DB->set_h_hash()</a> method. If no hash function is specified, a default
|
||
function will be used. Any application-specified hash function must
|
||
take a reference to a <a href="../api_reference/C/db.html" class="olink">DB</a> object, a pointer to a byte string and
|
||
its length, as arguments and return an unsigned, 32-bit hash value.</p>
|
||
</div>
|
||
<div class="sect2" lang="en" xml:lang="en">
|
||
<div class="titlepage">
|
||
<div>
|
||
<div>
|
||
<h3 class="title"><a id="am_conf_h_nelem"></a>Hash table size</h3>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<p>When setting up the hash database, knowing the expected number of elements
|
||
that will be stored in the hash table is useful. This value can be used
|
||
by the Hash access method implementation to more accurately construct the
|
||
necessary number of buckets that the database will eventually require.</p>
|
||
<p>The anticipated number of elements in the hash table can be specified by
|
||
calling the <a href="../api_reference/C/dbset_h_nelem.html" class="olink">DB->set_h_nelem()</a> method. If not specified, or set too low,
|
||
hash tables will expand gracefully as keys are entered, although a slight
|
||
performance degradation may be noticed. In order for the estimated number
|
||
of elements 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> method
|
||
must also be called to set the page fill factor.</p>
|
||
</div>
|
||
</div>
|
||
<div class="navfooter">
|
||
<hr />
|
||
<table width="100%" summary="Navigation footer">
|
||
<tr>
|
||
<td width="40%" align="left"><a accesskey="p" href="bt_conf.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="am_conf.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
|
||
</tr>
|
||
<tr>
|
||
<td width="40%" align="left" valign="top">Btree access method specific configuration </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="h" href="index.html">Home</a>
|
||
</td>
|
||
<td width="40%" align="right" valign="top"> Heap access method specific configuration</td>
|
||
</tr>
|
||
</table>
|
||
</div>
|
||
</body>
|
||
</html>
|