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<title>Db::set_bt_compress()</title>
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<p>Library Version 11.2.5.2</p>
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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_compress()</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_compare.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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<div class="titlepage">
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<div>
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<h2 class="title" style="clear: both"><a id="dbset_bt_compress"></a>Db::set_bt_compress()</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_cxx.h>
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extern "C" {
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typedef int (*bt_compress_fcn_type)(DB *db, const DBT *prevKey,
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const DBT *prevData, const DBT *key, const DBT *data, DBT *dest);
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typedef int (*bt_decompress_fcn_typ)(DB *db, const DBT *prevKey,
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const DBT *prevData, DBT *compressed, DBT *destKey,
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DBT *destData);
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};
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int
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Db::set_bt_compress(bt_compress_fcn_type bt_compress_fcn,
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bt_decompress_fcn_type bt_decompress_fcn);</pre>
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<p>
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Set the Btree compression and decompression functions. The
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compression function is called whenever a key/data pair is added to
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the tree and the decompression function is called whenever data is
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requested from the tree.
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</p>
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<p>
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This method is only compatible with prefix-based compression
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routines. This callback is mostly intended for compressing keys.
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From a performance perspective, it is better to perform compression
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of the data portion of your records outside of the Berkeley DB
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library.
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</p>
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<p>
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If NULL function pointers are specified, then default compression
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and decompression functions are used. Berkeley DB's default
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compression function performs prefix compression on all keys and
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prefix compression on data values for duplicate keys. If using
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default compression, both the default compression and decompression
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functions must be used.
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</p>
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<p>
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The <code class="methodname">Db::set_bt_compress()</code> method
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configures operations performed using the specified <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> handle, not all operations
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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_compress()</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_compress()</code> must be the same as
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that 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_compress()</code> <span>
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<span>
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method either returns a non-zero error value or throws an
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exception that encapsulates a non-zero error value on
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failure, and returns 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="id3572331"></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="id3572320"></a>bt_compress_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_compress_fcn</strong></span> function is the
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application-specified Btree compression function. The compression
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function takes six 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">prevKey</code>
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</p>
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<p>
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The <span class="bold"><strong>prevKey</strong></span>
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parameter is the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the key immediately preceding
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the application supplied 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">prevData</code>
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</p>
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<p>
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The <span class="bold"><strong>prevData</strong></span>
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parameter is the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the data associated with
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<span class="bold"><strong>prevKey</strong></span>.
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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">key</code>
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</p>
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<p>
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The <span class="bold"><strong>key</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the application supplied 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">data</code>
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</p>
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<p>
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The <span class="bold"><strong>data</strong></span> parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> representing the application
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supplied data.
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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">dest</code>
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</p>
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<p>
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The <span class="bold"><strong>dest</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the data stored in the tree,
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where the function should write the
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compressed data.
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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_compress_fcn</strong></span>
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function must return 0 on success and a non-zero value
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on failure. If the compressed data cannot fit in
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<span class="bold"><strong>dest->set_data()</strong></span>
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(the size of which is
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<span>
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returned by
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<span class="bold"><strong>dest->get_ulen()</strong></span>),
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</span>
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the function should identify the required buffer size in
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<span class="bold"><strong>dest->set_size()</strong></span>
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and return <code class="literal">DB_BUFFER_SMALL</code>.
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</p>
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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="id3572572"></a>bt_decompress_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_decompress_fcn</strong></span> function is the
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application-specified Btree decompression function. The decompression
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function takes six 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">prevKey</code>
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</p>
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<p>
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The <span class="bold"><strong>prevKey</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the key immediately preceding
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the key being decompressed.
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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">prevData</code>
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</p>
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<p>
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The <span class="bold"><strong>prevData</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the data associated with
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<span class="bold"><strong>prevKey</strong></span>.
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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">compressed</code>
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</p>
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<p>
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The
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<span class="bold"><strong>compressed</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>
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representing the data stored in the tree,
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that is, the compressed data.
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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">destKey</code>
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</p>
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<p>
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The <span class="bold"><strong>key</strong></span>
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parameter is the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> where
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the decompression function should store the
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decompressed 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">destData</code>
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</p>
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<p>
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The <span class="bold"><strong>data</strong></span>
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parameter is the
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<a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a> where
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the decompression function should store the
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decompressed 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_decompress_fcn</strong></span>
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function must return 0 on success and a non-zero value
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on failure.
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If the decompressed data cannot fit in
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<span class="bold"><strong>key->set_data()</strong></span>
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or
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<span class="bold"><strong>data->set_data()</strong></span>
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(the size of which is
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<span>
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returned by the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>'s
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<span class="bold"><strong>get_ulen()</strong></span> method),
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</span>
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the function should identify the required buffer size
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<span>
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using the <a class="link" href="dbt.html" title="Chapter 4. The Dbt Handle">Dbt</a>'s
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<span class="bold"><strong>set_size()</strong></span>
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method
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</span>
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and return <code class="literal">DB_BUFFER_SMALL</code>.
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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="id3572790"></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_compress()</code> <span>
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<span>
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method may fail and throw a <a class="link" href="dbexception.html" title="Chapter 6. The DbException Class">DbException</a>
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exception, encapsulating one of the following non-zero errors, or return one
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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="id3572539"></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="id3572781"></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="id3572788"></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_compare.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_bt_minkey.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_compare() </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_bt_minkey()</td>
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