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<!--$Id: db_set_bt_prefix.so,v 10.25 2000/09/08 21:35:26 bostic Exp $-->
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<h1>Db::set_bt_prefix</h1>
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<a href="../api_cxx/cxx_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
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<h3><pre>
#include <db_cxx.h>
<p>
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></h3>
<h1>Description</h1>
<p>Set the Btree prefix function. The prefix function must return the
number of bytes of the second key argument that would be required by
the Btree key comparison function to determine the second key argument's
ordering relationship with respect to the first key argument. 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 <b>data</b> and
<b>size</b> fields of the <a href="../api_cxx/dbt_class.html">Dbt</a> are the only fields that may be
used for the purposes of this determination.
<p>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="../ref/am_conf/bt_prefix.html">Btree
prefix comparison</a> section of the Reference Guide for more details about
how this works. The usefulness of this is data dependent, but in some
data sets can produce significantly reduced tree sizes and search times.
<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 key comparison
function.
<p>The Db::set_bt_prefix interface may only be used to configure Berkeley DB before
the <a href="../api_cxx/db_open.html">Db::open</a> interface is called.
<p>The Db::set_bt_prefix 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.
<h1>Errors</h1>
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>Called after <a href="../api_cxx/db_open.html">Db::open</a> was called.
</dl>
<h3>Class</h3>
<a href="../api_cxx/db_class.html">Db</a>
<h1>See Also</h1>
<a href="../api_cxx/db_close.html">Db::close</a>,
<a href="../api_cxx/db_cursor.html">Db::cursor</a>,
<a href="../api_cxx/db_del.html">Db::del</a>,
<a href="../api_cxx/db_err.html">Db::err</a>,
<a href="../api_cxx/db_fd.html">Db::fd</a>,
<a href="../api_cxx/db_get.html">Db::get</a>,
<a href="../api_cxx/db_get_byteswapped.html">Db::get_byteswapped</a>,
<a href="../api_cxx/db_get_type.html">Db::get_type</a>,
<a href="../api_cxx/db_join.html">Db::join</a>,
<a href="../api_cxx/db_key_range.html">Db::key_range</a>,
<a href="../api_cxx/db_open.html">Db::open</a>,
<a href="../api_cxx/db_put.html">Db::put</a>,
<a href="../api_cxx/db_remove.html">Db::remove</a>,
<a href="../api_cxx/db_set_bt_compare.html">Db::set_bt_compare</a>,
<a href="../api_cxx/db_set_bt_minkey.html">Db::set_bt_minkey</a>,
<a href="../api_cxx/db_set_bt_prefix.html">Db::set_bt_prefix</a>,
<a href="../api_cxx/db_set_cachesize.html">Db::set_cachesize</a>,
<a href="../api_cxx/db_set_dup_compare.html">Db::set_dup_compare</a>,
<a href="../api_cxx/db_set_errcall.html">Db::set_errcall</a>,
<a href="../api_cxx/db_set_errfile.html">Db::set_errfile</a>,
<a href="../api_cxx/db_set_errpfx.html">Db::set_errpfx</a>,
<a href="../api_cxx/db_set_flags.html">Db::set_flags</a>,
<a href="../api_cxx/db_set_h_ffactor.html">Db::set_h_ffactor</a>,
<a href="../api_cxx/db_set_h_hash.html">Db::set_h_hash</a>,
<a href="../api_cxx/db_set_h_nelem.html">Db::set_h_nelem</a>,
<a href="../api_cxx/db_set_lorder.html">Db::set_lorder</a>,
<a href="../api_cxx/db_set_malloc.html">Db::set_malloc</a>,
<a href="../api_cxx/db_set_pagesize.html">Db::set_pagesize</a>,
<a href="../api_cxx/db_set_paniccall.html">Db::set_paniccall</a>,
<a href="../api_cxx/db_set_q_extentsize.html">Db::set_q_extentsize</a>,
<a href="../api_cxx/db_set_realloc.html">Db::set_realloc</a>,
<a href="../api_cxx/db_set_re_delim.html">Db::set_re_delim</a>,
<a href="../api_cxx/db_set_re_len.html">Db::set_re_len</a>,
<a href="../api_cxx/db_set_re_pad.html">Db::set_re_pad</a>,
<a href="../api_cxx/db_set_re_source.html">Db::set_re_source</a>,
<a href="../api_cxx/db_stat.html">Db::stat</a>,
<a href="../api_cxx/db_sync.html">Db::sync</a>,
<a href="../api_cxx/db_upgrade.html">Db::upgrade</a>
and
<a href="../api_cxx/db_verify.html">Db::verify</a>.
</tt>
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