109 lines
7.8 KiB
HTML
109 lines
7.8 KiB
HTML
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Plan Cache" />
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<meta name="abstract" content="The Plan Cache is a repository that contains the access plans for queries that were optimized by SQE." />
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<meta name="description" content="The Plan Cache is a repository that contains the access plans for queries that were optimized by SQE." />
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<title>Plan Cache</title>
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</head>
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<body id="rzajqcache"><a name="rzajqcache"><!-- --></a>
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<img src="./delta.gif" alt="Start of change" /><!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Plan Cache</h1>
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<div><p>The Plan Cache is a repository that contains the access plans for
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queries that were optimized by SQE.</p>
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<p>Access plans generated by CQE are not stored in the Plan Cache; instead,
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they are stored in SQL Packages, the system-wide statement cache, and job
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cache). The purposes of the Plan Cache are to:</p>
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<ul><li>Facilitate the reuse of a query access plan when the same query is re-executed</li>
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<li>Store runtime information for subsequent use in future query optimizations</li>
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</ul>
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<p>Once an access plan is created, it is available for use by all users and
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all queries, regardless of where the query originates. Furthermore, when an
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access plan is tuned, when creating an index for example, all queries can
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benefit from this updated access plan. This eliminates the need to reoptimize
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the query, resulting in greater efficiency.</p>
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<p>The graphic below shows the concept of reusability of the query access
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plans stored in the Plan Cache:</p>
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<p><br /><img src="rzajq550.gif" alt="Description of the Plan Cache. Statements are stored from different packages and programs in different plans in the plan cache. If Statement 3 exists in both SQL package 1 and SQL package 2, the plan is stored once in the plan cache." /><br /></p>
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<p>As shown above, the Plan Cache is interrogated each time a query is executed
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in order to determine if a valid access plan exists that satisfies the requirements
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of the query. If a valid access plan is found, it is used to implement the
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query. Otherwise a new access plan is created and stored in the Plan Cache
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for future use. The Plan Cache is automatically updated with new query access
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plans as they are created, or is updated for an existing plan (the next time
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the query is run) when new statistics or indexes become available. The Plan
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Cache is also automatically updated by the database with runtime information
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as the queries are run. It is created with an overall size of 512 Megabytes
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(MB). Each plan cache entry contains the original query, the optimized query
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access plan and cumulative runtime information gathered during the runs of
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the query. In addition, several instances of query runtime objects are stored
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with a plan cache entry. These runtime objects are the real executables and
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temporary storage containers (hash tables, sorts, temporary indexes, and so
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on) used to run the query. All systems are currently configured with the same
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size Plan Cache, regardless of the server size or the hardware configuration. </p>
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<p>When the Plan Cache exceeds its designated size, a background task is automatically
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scheduled to remove plans from the Plan Cache. Access plans are deleted based
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upon the age of the access plan, how frequently it is being used and how much
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cumulative resources (CPU/IO) were consumed by the runs of the query. The
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total number of access plans stored in the Plan Cache depends largely upon
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the complexity of the SQL statements that are being executed. In certain test
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environments, there have been typically between 10,000 to 20,000 unique access
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plans stored in the Plan Cache. The Plan Cache is cleared when a system Initial
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Program Load (IPL) is performed.</p>
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<p>Multiple access plans can be maintained for a single SQL statement. Although
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the SQL statement itself is the primary hash key to the Plan Cache, different
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environmental settings can cause different access plans to be stored in the
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Plan Cache. Examples of these environmental settings include: </p>
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<ul><li>Different SMP Degree settings for the same query</li>
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<li>Different library lists specified for the query tables</li>
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<li>Different settings for the job's share of available memory in the current
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pool</li>
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<li>Different ALWCPYDTA settings</li>
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</ul>
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<p>Currently, the Plan Cache can maintain a maximum of 3 different access
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plans for the same SQL statement. As new access plans are created for the
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same SQL statement, older access plans are discarded to make room for the
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new access plans. There are, however, certain conditions that can cause an
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existing access plan to be invalidated. Examples of these include: </p>
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<ul><li>Specifying REOPTIMIZE_ACCESS_PLAN(*YES) or (*FORCE) in the QAQQINI table
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or in Run SQL Scripts</li>
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<li>Deleting or recreating the table that the access plan refers to</li>
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<li>Deleting an index that is used by the access plan</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="queryoptimize.htm" title="DB2 UDB for iSeries provides two query engines to process queries: the Classic Query Engine (CQE) and the SQL Query Engine (SQE).">Query Engine Overview</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="ipuc.htm" title="Some complex queries can perform better by using a sort or hashing method to evaluate the query instead of using or creating an index.">Effects of the ALWCPYDTA parameter on database performance</a></div>
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<div><a href="cqa.htm" title="You can modify different types of attributes of the queries that you will execute during a certain job with the Change Query Attributes (CHGQRYA) CL command, or by using the iSeries Navigator Change Query Attributes interface.">Change the attributes of your queries with the Change Query Attributes (CHGQRYA) command</a></div>
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<div><a href="rzajqdisplayplancache.htm" title="The Plan Cache contains a wealth of information about the SQE queries being run through the database. Its contents are viewable through the iSeries Navigator GUI interface.">Viewing the plan cache with iSeries Navigator</a></div>
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</div>
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</div>
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<img src="./deltaend.gif" alt="End of change" /></body>
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</html> |