mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
199 lines
8.6 KiB
HTML
199 lines
8.6 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Opening multiple databases in a single file</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="am.html" title="Chapter 3. Access Method Operations" />
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<link rel="prev" href="am.html" title="Chapter 3. Access Method Operations" />
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<link rel="next" href="am_partition.html" title="Partitioning databases" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Opening multiple databases in a
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single file</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="am.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Access Method Operations </th>
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<td width="20%" align="right"> <a accesskey="n" href="am_partition.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="am_opensub"></a>Opening multiple databases in a
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single file</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="am_opensub.html#idm140654540976064">Configuring databases sharing a file</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="am_opensub.html#idm140654546210656">Caching databases sharing a file</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="am_opensub.html#idm140654546233792">Locking in databases based on sharing a file</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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Applications may create multiple databases within a single
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physical file. This is useful when the databases are both
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numerous and reasonably small, in order to avoid creating a
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large number of underlying files, or when it is desirable to
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include secondary index databases in the same file as the
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primary index database. Putting multiple databases in a single
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physical file is an administrative convenience and unlikely to
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affect database performance.
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</p>
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<p>
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To open or create a file that will include more than a
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single database, specify a database name when calling the
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<a href="../api_reference/C/dbopen.html" class="olink">DB->open()</a> method.
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</p>
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<p>
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Physical files do not need to be comprised of a single type
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of database, and databases in a file may be of any mixture of
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types, except for Queue and Heap databases. Queue and Heap
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databases must be created one per file and cannot share a file
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with any other database type. There is no limit on the number
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of databases that may be created in a single file other than
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the standard Berkeley DB file size and disk space
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limitations.
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</p>
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<p>
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It is an error to attempt to open a second database in a
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file that was not initially created using a database name,
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that is, the file must initially be specified as capable of
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containing multiple databases for a second database to be
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created in it.
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</p>
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<p>
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It is not an error to open a file that contains multiple
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databases without specifying a database name, however the
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database type should be specified as DB_UNKNOWN and the
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database must be opened read-only. The handle that is returned
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from such a call is a handle on a database whose key values
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are the names of the databases stored in the database file and
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whose data values are opaque objects. No keys or data values
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may be modified or stored using this database handle.
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</p>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idm140654540976064"></a>Configuring databases sharing a file</h3>
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</div>
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</div>
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</div>
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<p>
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There are four pieces of configuration information which
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must be specified consistently for all databases in a
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file, rather than differing on a per-database basis. They
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are: byte order, checksum and encryption behavior, and
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page size. When creating additional databases in a file,
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any of these configuration values specified must be
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consistent with the existing databases in the file or an
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error will be returned.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idm140654546210656"></a>Caching databases sharing a file</h3>
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</div>
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</div>
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</div>
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<p>
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When storing multiple databases in a single physical
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file rather than in separate files, if any of the
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databases in a file is opened for update, all of the
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databases in the file must share a memory pool. In other
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words, they must be opened in the same database
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environment. This is so per-physical-file information
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common between the two databases is updated
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correctly.
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</p>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idm140654546233792"></a>Locking in databases based on sharing a file</h3>
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</div>
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</div>
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</div>
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<p>
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If databases are in separate files (and access to each
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separate database is single-threaded), there is no reason
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to perform any locking of any kind, and the two databases
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may be read and written simultaneously. Further, there
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would be no requirement to create a shared database
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environment in which to open those two databases.
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</p>
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<p>
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However, since multiple databases in a file exist in a
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single physical file, opening two databases in the same
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file simultaneously requires locking be enabled, unless
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all of the databases are read-only. As the locks for the
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two databases can only conflict during page allocation,
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this additional locking is unlikely to affect performance.
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The exception is when Berkeley DB Concurrent Data Store is
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configured; a single lock is used for all databases in the
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file when Berkeley DB Concurrent Data Store is configured,
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and a write to one database will block all accesses to all
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databases.
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</p>
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<p>
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In summary, programmers writing applications that open
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multiple databases in a single file will almost certainly
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need to create a shared database environment in the
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application as well. For more information on database
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environments, see <a class="xref" href="env.html#env_intro" title="Database environment introduction">Database environment introduction</a>
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</p>
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</div>
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</div>
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<div class="navfooter">
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||
<hr />
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||
<table width="100%" summary="Navigation footer">
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||
<tr>
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||
<td width="40%" align="left"><a accesskey="p" href="am.html">Prev</a> </td>
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||
<td width="20%" align="center">
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<a accesskey="u" href="am.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="am_partition.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 3. Access Method Operations </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Partitioning databases</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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