75 lines
4.7 KiB
HTML
75 lines
4.7 KiB
HTML
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<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en-US" xml:lang="en-us">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="dc.language" scheme="rfc1766" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<meta name="dc.date" scheme="iso8601" content="2005-09-19" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="security" content="public" />
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<meta name="Robots" content="index,follow"/>
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="keywords" content="sequences" />
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<title>Sequences</title>
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<link rel="stylesheet" type="text/css" href="ibmidwb.css" />
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<link rel="stylesheet" type="text/css" href="ic.css" />
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</head>
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<body>
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<a id="Top_Of_Page" name="Top_Of_Page"></a><!-- Java sync-link -->
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<script language = "Javascript" src = "../rzahg/synch.js" type="text/javascript"></script>
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<a name="sequences"></a>
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<h2 id="sequences"><a href="rbafzmst02.htm#ToC_42">Sequences</a></h2><a id="idx111" name="idx111"></a>
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<p>A sequence is a stored object that simply generates a sequence of numbers
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in a monotonically ascending (or descending) order. Sequences provide a way
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to have the database manager automatically generate unique integer and decimal
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primary keys, and to coordinate keys across multiple rows and tables. A sequence
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can be used to exploit parallelization, instead of programmatically generating
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unique numbers by locking the most recently used value and then incrementing
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it.</p>
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<p>Sequences are ideally suited to the task of generating unique key values.
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One sequence can be used for many tables, or a separate sequence can be created
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for each table requiring generated keys. A sequence has the following properties:</p>
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<ul>
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<li>Can have guaranteed, unique values, assuming that the sequence is not
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reset and does not allow the values to cycle.</li>
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<li>Can have increasing or decreasing values within a defined range.</li>
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<li>Can have an increment value other than 1 between consecutive values (the
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default is 1).</li>
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<li>Is recoverable.</li></ul>
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<p>Values for a given sequence are automatically generated by the database
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manager. Use of a sequence in the database avoids the performance bottleneck
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that results when an application implements sequences outside the database.
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The counter for the sequence is incremented (or decremented) independently
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from the transaction.</p>
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<p>In some cases, gaps can be introduced in a sequence. A gap can occur when
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a given transaction increments a sequence two times. The transaction may see
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a gap in the two numbers that are generated because there may be other transactions
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concurrently incrementing the same sequence. A user may not realize that other
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users are drawing from the same sequence. Furthermore, it is possible that
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a given sequence can appear to have generated gaps in the numbers, because
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a transaction that may have generated a sequence number may have rolled back.
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Updating a sequence is not part of a transaction's unit of recovery.</p>
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<p>A sequence is created with a CREATE SEQUENCE statement. A sequence can
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be referenced using a <span class="italic">sequence-reference</span>. A sequence
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reference can appear most places that an expression can appear. A sequence
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reference can specify whether the value to be returned is a newly generated
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value, or the previously generated value. For more information, see <a href="rbafzmstcsequence.htm#csequence">CREATE SEQUENCE</a> and <a href="rbafzmstdatetimearith.htm#seqref">Sequence reference</a>.</p>
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<p>Although there are similarities, a sequence is different than an identity
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column. A sequence is an object, whereas an identity column is a part of a
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table. A sequence can be used with multiple tables, but an identity column
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is part of a single table.</p>
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<hr /><br />
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[ <a href="#Top_Of_Page">Top of Page</a> | <a href="rbafzmstch1routines.htm">Previous Page</a> | <a href="rbafzmstauthown.htm">Next Page</a> | <a href="rbafzmst02.htm#wq1">Contents</a> |
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<a href="rbafzmstindex.htm#index">Index</a> ]
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<a id="Bot_Of_Page" name="Bot_Of_Page"></a>
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</body>
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</html>
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