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88 lines
4.4 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>Chapter 20. Sequences</title>
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="prev" href="txn_limits.html" title="Transaction limits" />
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<div class="libver">
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<p>Library Version 11.2.5.3</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">Chapter 20.
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Sequences
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="txn_limits.html">Prev</a> </td>
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<td width="20%" align="right"> <a accesskey="n" href="tcl.html">Next</a></td>
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<div class="chapter" 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"><a id="sequence"></a>Chapter 20.
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Sequences
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</h2>
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</div>
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</div>
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</div>
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<p>Sequences provide an arbitrary number of persistent objects that return
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an increasing or decreasing sequence of integers. Opening a sequence
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handle associates it with a record in a database. The handle can
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maintain a cache of values from the database so that a database update
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is not needed as the application allocates a value.</p>
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<p>A sequence is stored as a record pair in a database. The database may
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be of any type, but may not have been configured to support duplicate
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data items. The sequence is referenced by the key used when the
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sequence is created, therefore the key must be compatible with the
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underlying access method. If the database stores fixed-length records,
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the record size must be at least 64 bytes long.</p>
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<p>Since a sequence handle is opened using a database handle, the use of
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transactions with the sequence must follow how the database handle was
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opened. In other words, if the database handle was opened within a
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transaction, operations on the sequence handle must use transactions.
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Of course, if sequences are cached, not all operations will actually
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trigger a transaction.</p>
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<p>For the highest concurrency, caching should be used and the
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<a href="../api_reference/C/envset_flags.html#envset_flags_DB_AUTO_COMMIT" class="olink">DB_AUTO_COMMIT</a> and <a href="../api_reference/C/envset_flags.html#envset_flags_DB_TXN_NOSYNC" class="olink">DB_TXN_NOSYNC</a> flags should be
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specified to the <a href="../api_reference/C/seqget.html" class="olink">DB_SEQUENCE->get()</a> method call. If the allocation of the
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sequence value must be part of a transaction, and rolled back if the
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transaction aborts, then no caching should be specified and the
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transaction handle must be passed to the <a href="../api_reference/C/seqget.html" class="olink">DB_SEQUENCE->get()</a> method.</p>
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<p>For more information on the operations supported by the sequence handle, see the <a href="../api_reference/C/seq.html#seqlist" class="olink">Sequences and Related Methods</a> section in the <em class="citetitle">Berkeley DB C API Reference Guide.</em> </p>
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<td width="40%" align="right"> <a accesskey="n" href="tcl.html">Next</a></td>
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<tr>
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<td width="40%" align="left" valign="top">Transaction limits </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"> Chapter 21.
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Berkeley DB Extensions: Tcl
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</body>
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