1<!--$Id: db_set_bt_prefix.so,v 10.48 2004/08/13 03:38:56 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB: Db::set_bt_prefix</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<table width="100%"><tr valign=top> 12<td> 13<b>Db::set_bt_prefix</b> 14</td> 15<td align=right> 16<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a> 17<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db_cxx.h> 23<p> 24extern "C" { 25 typedef size_t (*bt_prefix_fcn_type)(DB *, const DBT *, const DBT *); 26}; 27int 28Db::set_bt_prefix(bt_prefix_fcn_type bt_prefix_fcn); 29</pre></b> 30<hr size=1 noshade> 31<b>Description: Db::set_bt_prefix</b> 32<p>Set the Btree prefix function. The prefix function is used to determine 33the amount by which keys stored on the Btree internal pages can be 34safely truncated without losing their uniqueness. See the 35<a href="../ref/am_conf/bt_prefix.html">Btree prefix comparison</a> section 36of the Berkeley DB Reference Guide for more details about how this works. The 37usefulness of this is data-dependent, but can produce significantly 38reduced tree sizes and search times in some data sets.</p> 39<p>If no prefix function or key comparison function is specified by the 40application, a default lexical comparison function is used as the prefix 41function. If no prefix function is specified and a key comparison 42function is specified, no prefix function is used. It is an error to 43specify a prefix function without also specifying a Btree key comparison 44function.</p> 45<p>The Db::set_bt_prefix method configures operations performed using the specified 46<a href="../api_cxx/db_class.html">Db</a> handle, not all operations performed on the underlying 47database.</p> 48<p>The Db::set_bt_prefix method may not be called after the <a href="../api_cxx/db_open.html">Db::open</a> method is called. 49If the database already exists when 50<a href="../api_cxx/db_open.html">Db::open</a> is called, the information specified to Db::set_bt_prefix must 51be the same as that historically used to create the database or 52corruption can occur.</p> 53<p>The Db::set_bt_prefix method 54either returns a non-zero error value 55or throws an exception that encapsulates a non-zero error value on 56failure, and returns 0 on success. 57</p> 58<b>Parameters</b> <br> 59 <b>bt_prefix_fcn</b><ul compact><li>The <b>bt_prefix_fcn</b> function is the application-specific Btree 60prefix function. The prefix function takes three parameters: 61<br> 62<b><b>db</b></b><ul compact><li>The <b>db</b> parameter is the enclosing database handle.</ul> 63<b><b>dbt1</b></b><ul compact><li>The <b>dbt1</b> parameter is a <a href="../api_cxx/dbt_class.html">Dbt</a> representing a database key.</ul> 64<b><b>dbt2</b></b><ul compact><li>The <b>dbt2</b> parameter is a <a href="../api_cxx/dbt_class.html">Dbt</a> representing a database key.</ul> 65<br> 66<p>The <b>bt_prefix_fcn</b> function must return the number of bytes of 67the second key parameter that would be required by the Btree key 68comparison function to determine the second key parameter's ordering 69relationship with respect to the first key parameter. If the two keys 70are equal, the key length should be returned. The prefix function must 71correctly handle any key values used by the application (possibly 72including zero-length keys). The <b>data</b> and <b>size</b> fields 73of the <a href="../api_cxx/dbt_class.html">Dbt</a> are the only fields that may be used for the purposes 74of this determination, and no particular alignment of the memory to 75which the <b>data</b> field refers may be assumed.</p></ul> 76<br> 77<br><b>Errors</b> 78<p>The Db::set_bt_prefix method 79may fail and throw 80<a href="../api_cxx/except_class.html">DbException</a>, 81encapsulating one of the following non-zero errors, or return one of 82the following non-zero errors:</p> 83<br> 84<b>EINVAL</b><ul compact><li>If the method was called after <a href="../api_cxx/db_open.html">Db::open</a> was called; or if an 85invalid flag value or parameter was specified.</ul> 86<br> 87<hr size=1 noshade> 88<br><b>Class</b> 89<a href="../api_cxx/db_class.html">Db</a> 90<br><b>See Also</b> 91<a href="../api_cxx/db_list.html">Databases and Related Methods</a> 92</tt> 93<table width="100%"><tr><td><br></td><td align=right> 94<a href="../api_cxx/api_core.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a> 95</td></tr></table> 96<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 97</body> 98</html> 99
