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87 lines
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<title>Reusable open data paths 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">Reusable open data paths with DB2<sup>®</sup> Multisystem</h1>
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<div><p>Reusable open data paths (ODPs) have special considerations for
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distributed queries. Like most other aspects of distributed queries, ODPs
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have two levels: distributed and local.</p>
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<p>The distributed ODP is the coordinating ODP. A distributed ODP associates
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the query to the user and controls the local ODPs. Local ODPs are located
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on each system involved in the query, and they take requests through the distributed
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ODP.</p>
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<div class="fignone" id="odp__odp1"><a name="odp__odp1"><!-- --></a><span class="figcap">Figure 1. Example of an ODP</span><br /><img src="rbal3507.gif" alt="Diagram of an Open Data Path (ODP). Your query is passed through the Distributed Coordinating ODP to be requested by the Local System ODP, System B ODP, and System C ODP." /><br /></div>
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<p>For example, if a request is made to perform an SQL FETCH, the request
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is made against the distributed ODP. The system then takes that request and
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performs the appropriate record retrieval against the local ODPs.</p>
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<div class="p">With distributed queries, it is possible for the distributed ODP to be
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reusable, yet for one or more of the local ODPs to be nonreusable; however,
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if the distributed query ODP is nonreusable, the local ODPs are always nonreusable.
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This is allowed so that: <ul><li>Each local system can decide the best way to open its local ODP (reusable
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versus nonreusable).</li>
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<li>Regardless of the local ODP methods, the distributed ODP can be opened
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as reusable as much as possible in order to maintain active resources, such
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as communications.</li>
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</ul>
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The system tries to make the distributed ODP reusable whenever possible,
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even when an underlying local ODP is not reusable. If this occurs, the system
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handles the ODP refresh as follows: <ul><li>Cycles through each local ODP</li>
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<li>Performs a refresh of a reusable local ODP</li>
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<li>Performs a <span class="q">"hard"</span> close and reopen of a nonreusable ODP</li>
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<li>Completes any remaining refresh of the distributed ODP itself that is
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needed</li>
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</ul>
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</div>
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<p>The distributed ODP is reusable more often than local ODPs,
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because the distributed ODP is not affected by some of the things that make
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local ODPs nonreusable, such as a host variable in a LIKE clause or the optimizer
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choosing nonreusable so that an index-from-index create operation can be performed.
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The cases that would make distributed ODPs nonreusable are a subset of those
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that affect local ODPs. This subset includes the following items:</p>
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<ul><li>The use of temporary files other than for sorting. These are called multistep
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distributed queries, and the optimizer debug message CPI4343 is signalled
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for these cases.</li>
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<li>Library list changes, which can affect the files being queried.</li>
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<li>OVRDBF changes, which affects the files being queried.</li>
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<li>Value changes for special registers USER or CURRENT TIMEZONE.</li>
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<li>Job CCSID changes.</li>
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<li>The Reclaim Resources (RCLRSC) command being issued.</li>
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</ul>
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<p>The reusability of the local ODP is affected by the same conditions that
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already exist for nondistributed query ODPs. Therefore, the same considerations
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apply to them as apply to local query ODPs.</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="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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