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3.6 KiB
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54 lines
3.6 KiB
HTML
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<!--$Id: bt_minkey.so,v 10.14 2000/03/18 21:43:08 bostic Exp $-->
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<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<html>
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<head>
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<title>Berkeley DB Reference Guide: Minimum keys per page</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,b+tree,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><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
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<td width="1%"><a href="../../ref/am_conf/bt_prefix.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/am_conf/bt_recnum.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p>
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<h1 align=center>Minimum keys per page</h1>
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<p>The number of keys stored on each page affects the size of a Btree and
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how it is maintained. Therefore, it also affects the retrieval and search
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performance of the tree. For each Btree, Berkeley DB computes a maximum key
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and data size. This size is a function of the page size and the fact that
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at least two key/data pairs must fit on any Btree page. Whenever key or
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data items exceed the calculated size, they are stored on overflow pages
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instead of in the standard Btree leaf pages.
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<p>Applications may use the <a href="../../api_c/db_set_bt_minkey.html">DB->set_bt_minkey</a> function to change the minimum
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number of keys that must fit on a Btree page from two to another value.
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Altering this value in turn alters the on-page maximum size, and can be
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used to force key and data items which would normally be stored in the
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Btree leaf pages onto overflow pages.
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<p>Some data sets can benefit from this tuning. For example, consider an
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application using large page sizes, with a data set almost entirely
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consisting of small key and data items, but with a few large items. By
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setting the minimum number of keys that must fit on a page, the
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application can force the outsized items to be stored on overflow pages.
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That in turn can potentially keep the tree more compact, that is, with
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fewer internal levels to traverse during searches.
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<p>The following calculation is similar to the one performed by the Btree
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implementation. (The <b>minimum_keys</b> value is multiplied by 2
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because each key/data pair requires 2 slots on a Btree page.)
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<p><blockquote><pre>maximum_size = page_size / (minimum_keys * 2)</pre></blockquote>
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<p>Using this calculation, if the page size is 8KB and the default
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<b>minimum_keys</b> value of 2 is used, then any key or data items
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larger than 2KB will be forced to an overflow page. If an application
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were to specify a <b>minimum_key</b> value of 100, then any key or data
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items larger than roughly 40 bytes would be forced to overflow pages.
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<p>It is important to remember that accesses to overflow pages do not perform
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as well as accesses to the standard Btree leaf pages, and so setting the
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value incorrectly can result in overusing overflow pages and decreasing
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the application's overall performance.
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<table><tr><td><br></td><td width="1%"><a href="../../ref/am_conf/bt_prefix.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/am_conf/bt_recnum.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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</body>
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</html>
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