86 lines
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86 lines
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<meta name="abstract" content="For the system to process the partitioned file in the most efficient manner, there are some tips you can consider when setting up or using a partitioning key." />
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<title>Choose partitioning keys with DB2 Multisystem</title>
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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">Choose partitioning keys with DB2 Multisystem</h1>
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<div><p>For the system to process the partitioned file in the most efficient
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manner, there are some tips you can consider when setting up or using a partitioning
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key.</p>
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<p>These tips are:</p>
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<ul><li>The best partitioning key is one that has many different values and, therefore,
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the partitioning activity results in an even distribution of the records of
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data. Customer numbers, last names, claim numbers, ZIP codes (regional mailing
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address codes), and telephone area codes are examples of good categories for
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using as partitioning keys. <p>Gender, because only two choices
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exist, male or female, is an example of a poor choice for a partitioning key.
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Gender causes too much data to be distributed to a single node instead of
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spread across the nodes. Also, when doing a query, gender as the partitioning
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key causes the system to process through too many records of data. It is inefficient;
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another field or fields of data can narrow the scope of the query and make
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it much more efficient. A partitioning key based on gender is a poor choice
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in cases where even distribution of data is wanted rather than
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distribution based on specific values.</p>
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<p>When preparing to change a local
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file into a distributed file, you can use the HASH function to get an idea
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of how the data is distributed. Because the HASH function can be used against
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local files and with any variety of columns, you can try different partitioning
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keys before actually changing the file to be distributed. For example, if
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you plan to use the ZIP code field of a file, you can run the HASH function
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using that field to get an idea of the number of records that HASH to each
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partition number. This helps you in choosing your partitioning key fields,
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or in creating the partition map in your node groups.</p>
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</li>
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<li>Do not choose a field that needs to be updated often. A restriction on
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partitioning key fields is that they can have their values updated only if
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the update does not force the record to a different node.</li>
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<li>Do not use many fields in the partitioning key; the best choice is to
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use one field. Using many fields forces the system to do more work at I/O
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time.</li>
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<li>Choose a simple data type, such as fixed-length character or integer,
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as your partitioning key. This consideration might help performance because
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the hashing is done for a single field of a simple data type.</li>
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<li>When choosing a partitioning key, you should consider the join and grouping
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criteria of the queries you typically run. For example, choosing a field that
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is never used as a join field for a file that is involved in joins can adversely
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affect join performance. See Query design for performance with DB2<sup>®</sup> Multisystem
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for information about running queries involving distributed files.</li>
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</ul>
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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="ption.htm" title="Partitioning is the process of distributing a file across the nodes in a node group.">Partitioning with DB2 Multisystem</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="qqp.htm" title="This topic provides you with some guidelines for designing queries so that they use query resources more efficiently when you run queries that use distributed files.">Query design for performance with DB2 Multisystem</a></div>
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