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<h2 class="title" style="clear: both"><a id="dbset_bt_prefix"></a>Db::set_bt_prefix()</h2>
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<pre class="programlisting">#include <db_cxx.h>
extern "C" {
typedef size_t (*bt_prefix_fcn_type)(DB *, const DBT *, const DBT *);
};
int
Db::set_bt_prefix(bt_prefix_fcn_type bt_prefix_fcn);</pre>
<p>
Set the Btree prefix function. The prefix function is used to
determine the amount by which keys stored on the Btree internal pages
can be safely truncated without losing their uniqueness. See the
<a href="../../programmer_reference/bt_conf.html#am_conf_bt_prefix" class="olink">Btree prefix comparison</a> section of the Berkeley DB Reference Guide for more
details about how this works. The usefulness of this is
data-dependent, but can produce significantly reduced tree sizes and
search times in some data sets.
</p>
<p>
If no prefix function or key comparison function is specified by the
application, a default lexical comparison function is used as the
prefix function. If no prefix function is specified and a key
comparison function is specified, no prefix function is used. It is
an error to specify a prefix function without also specifying a Btree
key comparison function.
</p>
<p>
The <code class="methodname">Db::set_bt_prefix()</code> method configures operations performed using
the specified <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle,
not all operations performed on the underlying database.
</p>
<p>
The <code class="methodname">Db::set_bt_prefix()</code> method may not be called after the
<a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method is called. If
the database already exists when <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
is called, the information specified to <code class="methodname">Db::set_bt_prefix()</code> must be the same as that
historically used to create the database or corruption can occur.
</p>
<p>
The <code class="methodname">Db::set_bt_prefix()</code> <span>
<span>
method either returns a non-zero error value or throws an
exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
</span>
</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
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<div>
<h3 class="title"><a id="id1644020"></a>Parameters</h3>
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<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1644687"></a>bt_prefix_fcn</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>bt_prefix_fcn</strong></span> function is the
application-specific Btree prefix function. The prefix function takes
three parameters:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">db</code>
</p>
<p>
The <span class="bold"><strong>db</strong></span> parameter is the enclosing
database handle.
</p>
</li>
<li>
<p>
<code class="literal">dbt1</code>
</p>
<p>
The <span class="bold"><strong>dbt1</strong></span> parameter is a
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing a database key.
</p>
</li>
<li>
<p>
<code class="literal">dbt2</code>
</p>
<p>
The <span class="bold"><strong>dbt2</strong></span> parameter is a
<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing a database key.
</p>
</li>
</ul>
</div>
<p>
The <span class="bold"><strong>bt_prefix_fcn</strong></span> function must return the
number of bytes of the second key parameter that would be required by the Btree
key comparison function to determine the second key parameter's ordering
relationship with respect to the first key parameter. If the two keys are equal,
the key length should be returned. The prefix function must correctly handle any
key values used by the application (possibly including zero-length keys). The
<span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span>
fields of the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> are the only fields
that may be used for the purposes of this determination, and no particular
alignment of the memory to which the <span class="bold"><strong>data</strong></span> field
refers may be assumed.
</p>
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<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1644983"></a>Errors</h3>
</div>
</div>
</div>
<p>
The <code class="methodname">Db::set_bt_prefix()</code> <span>
<span>
method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
</span>
</span>
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id1644866"></a>EINVAL</h4>
</div>
</div>
</div>
<p>
If the method was called after <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a>
was called; or if an invalid flag value or parameter was specified.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id1644867"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
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<div>
<div>
<h3 class="title"><a id="id1644902"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
</p>
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