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225 lines
9.7 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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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>DB->set_bt_prefix()</title>
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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 C API Reference" />
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<link rel="up" href="db.html" title="Chapter 2. The DB Handle" />
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<link rel="prev" href="dbset_bt_minkey.html" title="DB->set_bt_minkey()" />
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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 11.2.5.2</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">DB->set_bt_prefix()</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="dbset_bt_minkey.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The DB Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="dbset_cachesize.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="dbset_bt_prefix"></a>DB->set_bt_prefix()</h2>
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</div>
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</div>
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</div>
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<pre class="programlisting">#include <db.h>
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int
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DB->set_bt_prefix(DB *db,
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size_t (*bt_prefix_fcn)(DB *, const *dbt1, const *dbt2)); </pre>
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<p>
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Set the Btree prefix function. The prefix function is used to
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determine the amount by which keys stored on the Btree internal pages
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can be safely truncated without losing their uniqueness. See the
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<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
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details about how this works. The usefulness of this is
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data-dependent, but can produce significantly reduced tree sizes and
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search times in some data sets.
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</p>
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<p>
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If no prefix function or key comparison function is specified by the
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application, a default lexical comparison function is used as the
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prefix function. If no prefix function is specified and a key
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comparison function is specified, no prefix function is used. It is
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an error to specify a prefix function without also specifying a Btree
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key comparison function.
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</p>
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<p>
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The <code class="methodname">DB->set_bt_prefix()</code> method configures operations performed using
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the specified <a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a> handle,
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not all operations performed on the underlying database.
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</p>
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<p>
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The <code class="methodname">DB->set_bt_prefix()</code> method may not be called after the
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<a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a> method is called. If
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the database already exists when <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
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is called, the information specified to <code class="methodname">DB->set_bt_prefix()</code> must be the same as that
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historically used to create the database or corruption can occur.
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</p>
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<p>
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The <code class="methodname">DB->set_bt_prefix()</code> <span>
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<span>
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method returns a non-zero error value on failure and 0 on success.
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</span>
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</span>
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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="id3866266"></a>Parameters</h3>
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</div>
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</div>
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</div>
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<div class="sect3" 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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<h4 class="title"><a id="id3866270"></a>bt_prefix_fcn</h4>
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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 <span class="bold"><strong>bt_prefix_fcn</strong></span> function is the
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application-specific Btree prefix function. The prefix function takes
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three parameters:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="literal">db</code>
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</p>
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<p>
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The <span class="bold"><strong>db</strong></span> parameter is the enclosing
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database handle.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">dbt1</code>
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</p>
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<p>
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The <span class="bold"><strong>dbt1</strong></span> parameter is a
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<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> representing a database key.
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</p>
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</li>
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<li>
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<p>
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<code class="literal">dbt2</code>
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</p>
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<p>
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The <span class="bold"><strong>dbt2</strong></span> parameter is a
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<a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> representing a database key.
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</p>
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</li>
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</ul>
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</div>
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<p>
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The <span class="bold"><strong>bt_prefix_fcn</strong></span> function must return the
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number of bytes of the second key parameter that would be required by the Btree
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key comparison function to determine the second key parameter's ordering
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relationship with respect to the first key parameter. If the two keys are equal,
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the key length should be returned. The prefix function must correctly handle any
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key values used by the application (possibly including zero-length keys). The
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<span class="bold"><strong>data</strong></span> and <span class="bold"><strong>size</strong></span>
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fields of the <a class="link" href="dbt.html" title="Chapter 4. The DBT Handle">DBT</a> are the only fields
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that may be used for the purposes of this determination, and no particular
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alignment of the memory to which the <span class="bold"><strong>data</strong></span> field
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refers may be assumed.
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</p>
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</div>
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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="id3866345"></a>Errors</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 <code class="methodname">DB->set_bt_prefix()</code> <span>
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<span>
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method may fail and return one of the following non-zero errors:
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</span>
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</span>
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</p>
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<div class="sect3" 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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<h4 class="title"><a id="id3866544"></a>EINVAL</h4>
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</div>
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</div>
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</div>
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<p>
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If the method was called after <a class="xref" href="dbopen.html" title="DB->open()">DB->open()</a>
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was called; or if an invalid flag value or parameter was specified.
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</p>
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</div>
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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="id3866562"></a>Class</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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<a class="link" href="db.html" title="Chapter 2. The DB Handle">DB</a>
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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="id3866492"></a>See Also</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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<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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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="dbset_bt_minkey.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="db.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="dbset_cachesize.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">DB->set_bt_minkey() </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"> DB->set_cachesize()</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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