<!--$Id: db_set_bt_prefix.so,v 10.48 2004/08/13 03:38:56 bostic Exp $--> <!--Copyright (c) 1997,2007 Oracle. All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: Db::set_bt_prefix</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <table width="100%"><tr valign=top> <td> <h3>Db::set_bt_prefix</h3> </td> <td align=right> <a href="../../db46-devel-4.6.21/api_cxx/api_core.html"><img src="../../db46-devel-4.6.21/images/api.gif" alt="API"></a> <a href="../../db46-devel-4.6.21/ref/toc.html"><img src="../../db46-devel-4.6.21/images/ref.gif" alt="Ref"></a></td> </tr></table> <hr size=1 noshade> <tt> <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> <hr size=1 noshade> <h3>Description: Db::set_bt_prefix</h3> <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="../../db46-devel-4.6.21/ref/am_conf/bt_prefix.html">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 Db::set_bt_prefix method configures operations performed using the specified <a href="../../db46-devel-4.6.21/api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying database.</p> <p>The Db::set_bt_prefix method may not be called after the <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> method is called. If the database already exists when <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_bt_prefix must be the same as that historically used to create the database or corruption can occur.</p> <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. </p> <h3>Parameters</h3> <dl compact> <dt><b>bt_prefix_fcn</b><dd>The <b>bt_prefix_fcn</b> function is the application-specific Btree prefix function. The prefix function takes three parameters: <dl compact> <dt><b>db</b><dd>The <b>db</b> parameter is the enclosing database handle. <dt><b>dbt1</b><dd>The <b>dbt1</b> parameter is a <a href="../../db46-devel-4.6.21/api_cxx/dbt_class.html">Dbt</a> representing a database key. <dt><b>dbt2</b><dd>The <b>dbt2</b> parameter is a <a href="../../db46-devel-4.6.21/api_cxx/dbt_class.html">Dbt</a> representing a database key. </dl> <p>The <b>bt_prefix_fcn</b> 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 <b>data</b> and <b>size</b> fields of the <a href="../../db46-devel-4.6.21/api_cxx/dbt_class.html">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 <b>data</b> field refers may be assumed.</p> </dl> <h3>Errors</h3> <p>The Db::set_bt_prefix method may fail and throw <a href="../../db46-devel-4.6.21/api_cxx/except_class.html">DbException</a>, encapsulating one of the following non-zero errors, or return one of the following non-zero errors:</p> <dl compact> <dt>EINVAL<dd>If the method was called after <a href="../../db46-devel-4.6.21/api_cxx/db_open.html">Db::open</a> was called; or if an invalid flag value or parameter was specified. </dl> <hr size=1 noshade> <h3>Class</h3> <a href="../../db46-devel-4.6.21/api_cxx/db_class.html">Db</a> <h3>See Also</h3> <a href="../../db46-devel-4.6.21/api_cxx/db_list.html">Databases and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../../db46-devel-4.6.21/api_cxx/api_core.html"><img src="../../db46-devel-4.6.21/images/api.gif" alt="API"></a><a href="../../db46-devel-4.6.21/ref/toc.html"><img src="../../db46-devel-4.6.21/images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1>Copyright (c) 1996,2007 Oracle. 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