119 lines
7.6 KiB
HTML
119 lines
7.6 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Title" content="Effects of changing fields in a file description" />
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<meta name="abstract" content="The system uses the information in the record format description to determine the level identifier. Changes to the fields in a file description cause the level identifier to change. Changes in key fields or select/omit fields might cause unexpected results in programs using the new access path." />
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<meta name="description" content="The system uses the information in the record format description to determine the level identifier. Changes to the fields in a file description cause the level identifier to change. Changes in key fields or select/omit fields might cause unexpected results in programs using the new access path." />
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<meta name="DC.subject" content="field, changing in a file description, effects of" />
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<meta name="keywords" content="field, changing in a file description, effects of" />
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<meta name="DC.Relation" scheme="URI" content="rbafochgfile.htm" />
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<meta name="DC.Relation" scheme="URI" content="../cl/dspfd.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Effects of changing fields in a file description</title>
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</head>
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<body id="rbafoecffd"><a name="rbafoecffd"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Effects of changing fields in a file description</h1>
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<div><p> The system uses the information in the record format description
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to determine the level identifier. Changes to the fields in a file description
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cause the level identifier to change. Changes in key fields or select/omit
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fields might cause unexpected results in programs using the new access path. </p>
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<p>When a program that uses externally described data is compiled, the compiler
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copies the file descriptions of the files into the compiled program. When
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you run the program, the system can verify that the record formats the program
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was compiled with are the same as the record formats currently defined for
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the file. The default is to do level checking. The system assigns a unique
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level identifier for each record format when the file it is associated with
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is created. The system uses the information in the record format description
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to determine the level identifier. This information includes the total length
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of the record format, the record format name, the number and order of fields
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defined, the data type, the size of the fields, the field names, the number
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of decimal positions in the field, and whether the field allows the null value.
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Changes to this information in a record format cause the level identifier
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to change.</p>
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<div class="p">The following DDS information has no effect on the level identifier and,
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therefore, can be changed without recompiling the program that uses the file: <ul><li>TEXT keyword</li>
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<li>COLHDG keyword</li>
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<li>CHECK keyword</li>
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<li>EDTCDE keyword</li>
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<li>EDTWRD keyword</li>
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<li>REF keyword</li>
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<li>REFFLD keyword</li>
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<li>CMP, RANGE, and VALUES keywords</li>
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<li>TRNTBL keyword</li>
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<li>REFSHIFT keyword</li>
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<li>DFT keyword</li>
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<li>CCSID keyword</li>
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<li>Join specifications and join keywords</li>
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<li>Key fields</li>
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<li>Access path keywords</li>
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<li>Select/omit fields</li>
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</ul>
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</div>
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<p>Keep in mind that even though changing key fields or select/omit fields
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does not cause a level check, the change might cause unexpected results in
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programs using the new access path. For example, changing the key field from
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the customer number to the customer name changes the order in which the records
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are retrieved, and might cause unexpected problems in the programs processing
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the file.</p>
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<p>If level checking is specified (or defaulted to), the level identifier
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of the file to be used is compared to the level identifier of the file in
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your program when the file is opened. If the identifiers differ, a message
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is sent to the program to identify the changed condition and the changes might
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affect your program. You can compile your program again so that the changes
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are included.</p>
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<p>An alternative is to display the file description to determine if the changes
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affect your program. You can use the Display File Field Description (DSPFFD)
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command to display the description or, if you have source entry utility (SEU),
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you can display the source file containing the DDS for the file.</p>
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<p>The format level identifier defined in the file can be displayed by the
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Display File Description (DSPFD) command. When you are displaying
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the level identifier, remember that the record format identifier is compared,
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rather than the file identifier.</p>
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<p>Not every change in a file necessarily affects your program. For example,
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if you add a field to the end of a file and your program does not use the
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new field, you do not have to recompile your program. If the changes do not
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affect your program, you can use the Change Physical File (CHGPF) or the Change
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Logical File (CHGLF) command with LVLCHK(*NO) specified to turn off level
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checking for the file, or you can enter an Override with Database File (OVRDBF)
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command with LVLCHK(*NO) specified so that you can run your program without
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level checking.</p>
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<p>Keep in mind that level checking is the preferred method of operating.
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The use of LVLCHK(*YES) is a good database integrity practice. The results
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produced by LVLCHK(*NO) cannot always be predicted.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafochgfile.htm" title="These topics discuss how to change the database file descriptions and attributes. Considerations for changing fields in a file description are also included.">Change database file descriptions and attributes</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="../cl/dspffd.htm">Display File Field Description (DSPFFD) command</a></div>
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<div><a href="../cl/dspfd.htm">Display File Description (DSPFD) command</a></div>
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<div><a href="../cl/chgpf.htm">Change Physical File (CHGPF) command</a></div>
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<div><a href="../cl/chglf.htm">Change Logical File (CHGLF) command</a></div>
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<div><a href="../cl/ovrdbf.htm">Override with Database File (OVRDBF) command</a></div>
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</div>
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</div>
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</body>
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</html>
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