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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Chapter 2. Access Method Configuration</title>
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<link rel="next" href="am_conf_select.html" title="Selecting an access method" />
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<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Chapter 2. Access Method Configuration </th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="intro_products.html">Prev</a> </td>
<th width="60%" align="center"> </th>
<td width="20%" align="right"> <a accesskey="n" href="am_conf_select.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="am_conf"></a>Chapter 2. Access Method Configuration </h2>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="am_conf.html#am_conf_intro"> What are the available access
methods? </a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="am_conf.html#idm2727912">Btree</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1674312">Hash</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1985432">Heap</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1872368">Queue</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1924872">Recno</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="am_conf_select.html">Selecting an access method</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="am_conf_select.html#idm2274400">Btree or Heap?</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf_select.html#idm2915784">Hash or Btree?</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf_select.html#idm2732704">Queue or Recno?</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="am_conf_logrec.html">Logical record numbers</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="general_am_conf.html">General access method configuration</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="general_am_conf.html#am_conf_pagesize">Selecting a page size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="general_am_conf.html#am_conf_cachesize">Selecting a cache size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="general_am_conf.html#am_conf_byteorder">Selecting a byte order</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="general_am_conf.html#am_conf_dup">Duplicate data items</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="general_am_conf.html#am_conf_malloc">Non-local memory
allocation</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="bt_conf.html">Btree access method specific configuration</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="bt_conf.html#am_conf_bt_compare">Btree comparison</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bt_conf.html#am_conf_bt_prefix">Btree prefix
comparison</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bt_conf.html#am_conf_bt_minkey">Minimum keys per page</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bt_conf.html#am_conf_bt_recnum">Retrieving Btree records by logical record number</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="bt_conf.html#am_conf_bt_compress">Compression</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="hash_conf.html">Hash access method specific
configuration</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="hash_conf.html#am_conf_h_ffactor">Page fill factor</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="hash_conf.html#am_conf_h_hash">Specifying a database hash</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="hash_conf.html#am_conf_h_nelem">Hash table size</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="heap_conf.html">Heap access method specific
configuration</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rq_conf.html">Queue and Recno access method specific configuration</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_recno">Managing record-based
databases</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_extentsize">Selecting a Queue extent
size</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_re_source">Flat-text backing
files</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rq_conf.html#am_conf_renumber">Logically renumbering
records</a>
</span>
</dt>
</dl>
</dd>
</dl>
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="am_conf_intro"></a> What are the available access
methods? </h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="am_conf.html#idm2727912">Btree</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1674312">Hash</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1985432">Heap</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1872368">Queue</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_conf.html#idm1924872">Recno</a>
</span>
</dt>
</dl>
</div>
<p>
Berkeley DB currently offers five access methods: Btree,
Hash, Heap, Queue and Recno.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm2727912"></a>Btree</h3>
</div>
</div>
</div>
<p>
The Btree access method is an implementation of a
sorted, balanced tree structure. Searches, insertions, and
deletions in the tree all take
<span class="emphasis"><em>O(height)</em></span> time, where
<span class="emphasis"><em>height</em></span> is the number of levels in
the Btree from the root to the leaf pages. The upper bound
on the height is <span class="emphasis"><em>log base_b N</em></span>, where
<span class="emphasis"><em>base_b</em></span> is the smallest number of
keys on a page, and <span class="emphasis"><em>N</em></span> is the total
number of keys stored.
</p>
<p>
Inserting unordered data into a Btree can result in
pages that are only half-full. DB makes ordered (or
inverse ordered) insertion the best case, resulting in
nearly full-page space utilization.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1674312"></a>Hash</h3>
</div>
</div>
</div>
<p>
The Hash access method data structure is an
implementation of Extended Linear Hashing, as described in
"Linear Hashing: A New Tool for File and Table
Addressing", Witold Litwin, <span class="emphasis"><em>Proceedings of the
6th International Conference on Very Large Databases
(VLDB)</em></span>, 1980.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1985432"></a>Heap</h3>
</div>
</div>
</div>
<p>
The Heap access method stores records in a heap file.
Records are referenced solely by the page and offset at
which they are written. Because records are written in a
heap file, compaction is not necessary when deleting
records, which allows for more efficient use of space than
if Btree is in use. The Heap access method is intended for
platforms with constrained disk space, especially if those
systems are performing a great many record creation and
deletions.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1872368"></a>Queue</h3>
</div>
</div>
</div>
<p>
The Queue access method stores fixed-length records
with logical record numbers as keys. It is designed for
fast inserts at the tail and has a special cursor consume
operation that deletes and returns a record from the head
of the queue. The Queue access method uses record level
locking.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1924872"></a>Recno</h3>
</div>
</div>
</div>
<p>
The Recno access method stores both fixed and
variable-length records with logical record numbers as
keys, optionally backed by a flat text (byte stream) file.
</p>
</div>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="intro_products.html">Prev</a> </td>
<td width="20%" align="center"> </td>
<td width="40%" align="right"> <a accesskey="n" href="am_conf_select.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">The Berkeley DB products </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
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<td width="40%" align="right" valign="top"> Selecting an access method</td>
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