Import BSDDB 4.7.25 (as of svn r89086)
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Zachary Ware
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docs/api_c/db_set_bt_prefix.html
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docs/api_c/db_set_bt_prefix.html
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<!--$Id: db_set_bt_prefix.html 63573 2008-05-23 21:43:21Z trent.nelson $-->
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<!--Copyright (c) 1997,2008 Oracle. All rights reserved.-->
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<!--See the file LICENSE for redistribution information.-->
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<html>
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<head>
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<title>Berkeley DB: DB->set_bt_prefix</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++">
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</head>
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<body bgcolor=white>
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<table width="100%"><tr valign=top>
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<td>
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<b>DB->set_bt_prefix</b>
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</td>
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<td align=right>
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<a href="../api_c/api_core.html"><img src="../images/api.gif" alt="API"></a>
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<a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a></td>
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</tr></table>
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<hr size=1 noshade>
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<tt>
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<b><pre>
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#include <db.h>
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<p>
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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 DBT *, const DBT *));
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</pre></b>
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<hr size=1 noshade>
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<b>Description: DB->set_bt_prefix</b>
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<p>Set the Btree prefix function. The prefix function is used to determine
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the amount by which keys stored on the Btree internal pages can be
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safely truncated without losing their uniqueness. See the
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<a href="../ref/am_conf/bt_prefix.html">Btree prefix comparison</a> section
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of the Berkeley DB Reference Guide for more details about how this works. The
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usefulness of this is data-dependent, but can produce significantly
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reduced tree sizes and search times in some data sets.</p>
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<p>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 prefix
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function. If no prefix function is specified and a key comparison
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function is specified, no prefix function is used. It is an error to
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specify a prefix function without also specifying a Btree key comparison
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function.</p>
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<p>The DB->set_bt_prefix method configures operations performed using the specified
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<a href="../api_c/db_class.html">DB</a> handle, not all operations performed on the underlying
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database.</p>
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<p>The DB->set_bt_prefix method may not be called after the <a href="../api_c/db_open.html">DB->open</a> method is called.
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If the database already exists when
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<a href="../api_c/db_open.html">DB->open</a> is called, the information specified to DB->set_bt_prefix must
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be the same as that historically used to create the database or
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corruption can occur.</p>
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<p>The DB->set_bt_prefix method
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returns a non-zero error value on failure
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and 0 on success.
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</p>
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<b>Parameters</b> <br>
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<b>bt_prefix_fcn</b><ul compact><li>The <b>bt_prefix_fcn</b> function is the application-specific Btree
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prefix function. The prefix function takes three parameters:
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<br>
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<b><b>db</b></b><ul compact><li>The <b>db</b> parameter is the enclosing database handle.</ul>
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<b><b>dbt1</b></b><ul compact><li>The <b>dbt1</b> parameter is a <a href="../api_c/dbt_class.html">DBT</a> representing a database key.</ul>
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<b><b>dbt2</b></b><ul compact><li>The <b>dbt2</b> parameter is a <a href="../api_c/dbt_class.html">DBT</a> representing a database key.</ul>
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<br>
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<p>The <b>bt_prefix_fcn</b> function must return the number of bytes of
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the second key parameter that would be required by the Btree key
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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
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are equal, the key length should be returned. The prefix function must
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correctly handle any key values used by the application (possibly
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including zero-length keys). The <b>data</b> and <b>size</b> fields
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of the <a href="../api_c/dbt_class.html">DBT</a> are the only fields that may be used for the purposes
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of this determination, and no particular alignment of the memory to
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which the <b>data</b> field refers may be assumed.</p></ul>
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<br>
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<br><b>Errors</b>
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<p>The DB->set_bt_prefix method
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may fail and return one of the following non-zero errors:</p>
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<br>
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<b>EINVAL</b><ul compact><li>If the method was called after <a href="../api_c/db_open.html">DB->open</a> was called; or if an
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invalid flag value or parameter was specified.</ul>
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<br>
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<hr size=1 noshade>
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<br><b>Class</b>
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<a href="../api_c/db_class.html">DB</a>
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<br><b>See Also</b>
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<a href="../api_c/db_list.html">Databases and Related Methods</a>
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</tt>
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<table width="100%"><tr><td><br></td><td align=right>
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<a href="../api_c/api_core.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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</td></tr></table>
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<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font>
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</body>
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</html>
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