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Access Method Configuration
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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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<dt>
<span class="sect2">
<a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
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<span class="sect2">
<a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
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<span class="sect2">
<a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
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<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>
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<h3 class="title"><a id="am_conf_h_ffactor"></a>Page fill factor</h3>
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<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>
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<h3 class="title"><a id="am_conf_h_hash"></a>Specifying a database hash</h3>
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<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>
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<div>
<div>
<h3 class="title"><a id="am_conf_h_nelem"></a>Hash table size</h3>
</div>
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</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>
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