2011-09-13 17:44:24 +00:00
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<title>Hash access method specific configuration</title>
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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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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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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 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.
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Access Method Configuration
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
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<hr />
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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 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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</div>
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<p>
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There are a series of configuration tasks which you can perform when
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using the Hash access method. They are described 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>The density, or page fill factor, is an approximation of the number of
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keys allowed to accumulate in any one bucket, determining when the hash
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table grows or shrinks. If you know the average sizes of the keys and
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data in your data set, setting the fill factor can enhance performance.
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A reasonable rule to use to compute fill factor is:</p>
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<pre class="programlisting">(pagesize - 32) / (average_key_size + average_data_size + 8)</pre>
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<p>The desired density within the hash table can be specified by calling
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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
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be selected dynamically as pages are filled.</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>The database hash determines in which bucket a particular key will reside.
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The goal of hashing keys is to distribute keys equally across the database
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pages, therefore it is important that the hash function work well with
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the specified keys so that the resulting bucket usage is relatively
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uniform. A hash function that does not work well can effectively turn
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into a sequential list.</p>
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<p>No hash performs equally well on all possible data sets. It is possible
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that applications may find that the default hash function performs poorly
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with a particular set of keys. The distribution resulting from the hash
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function can be checked using the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility. By comparing the
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number of hash buckets and the number of keys, one can decide if the entries
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are hashing in a well-distributed manner.</p>
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<p>The hash function for the hash table can be specified by calling the
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<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
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function will be used. Any application-specified hash function must
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take a reference to a <a href="../api_reference/C/db.html" class="olink">DB</a> object, a pointer to a byte string and
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its length, as arguments and return an unsigned, 32-bit hash value.</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>When setting up the hash database, knowing the expected number of elements
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that will be stored in the hash table is useful. This value can be used
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by the Hash access method implementation to more accurately construct the
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necessary number of buckets that the database will eventually require.</p>
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<p>The anticipated number of elements in the hash table can be specified by
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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,
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hash tables will expand gracefully as keys are entered, although a slight
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performance degradation may be noticed. In order for the estimated number
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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
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must also be called to set the page fill factor.</p>
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</div>
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<a accesskey="u" href="am_conf.html">Up</a>
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<td width="40%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td>
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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 width="40%" align="right" valign="top"> Heap access method specific configuration</td>
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</tr>
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