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<h1>Db.set_bt_prefix</h1>
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<h3><pre>
import com.sleepycat.db.*;
<p>
public interface DbBtreePrefix
{
public abstract int bt_prefix(Db db, Dbt dbt1, Dbt dbt2);
}
public class Db
{
public void set_bt_prefix(DbBtreePrefix bt_prefix)
throws DbException;
...
}
</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_java/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>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 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>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 method configures operations performed using the specified
<a href="../api_java/db_class.html">Db</a> handle, not all operations performed on the underlying
database.
<p>The Db.set_bt_prefix interface may not be called after the <a href="../api_java/db_open.html">Db.open</a>
interface is called.
If the database already exists when
<a href="../api_java/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>The Db.set_bt_prefix method throws an exception that encapsulates a non-zero error value on
failure.
<h1>Errors</h1>
<p>The Db.set_bt_prefix method may fail and throw an exception encapsulating a non-zero error for the following conditions:
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>Called after <a href="../api_java/db_open.html">Db.open</a> was called.
</dl>
<p>The Db.set_bt_prefix method may fail and throw an exception for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the Db.set_bt_prefix method may fail and
throw a <a href="../api_java/runrec_class.html">DbRunRecoveryException</a>,
in which case all subsequent Berkeley DB calls will fail in the same way.
<h1>Class</h1>
<a href="../api_java/db_class.html">Db</a>
<h1>See Also</h1>
<a href="../api_java/db_list.html">Databases and Related Methods</a>
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