129 lines
8.6 KiB
HTML
129 lines
8.6 KiB
HTML
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<meta name="DC.Title" content="Manually collecting and refreshing statistics" />
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<meta name="abstract" content="You can manually collect and refresh statistics through iSeries Navigator or by using Statistics APIs." />
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<title>Manually collecting and refreshing statistics</title>
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<body id="statsmanually"><a name="statsmanually"><!-- --></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">Manually collecting and refreshing statistics</h1>
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<div><p>You can manually collect and refresh statistics through iSeries™ Navigator
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or by using Statistics APIs.</p>
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<div class="section"><p>To collect statistics using iSeries Navigator, right-click a table
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or alias and select Statistic Data. On the Statistic Data dialog, click New.
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Then select the columns that you want to collect statistics for. Once you
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have selected the columns, you can collect the statistics immediately or collect
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them in the background. </p>
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</div>
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<div class="section"><p>To refresh a statistic using iSeries Navigator, right-click a table
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or alias and select <span class="uicontrol">Statistic Data</span>. Click <span class="uicontrol">Update</span>.
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Select the statistic that you want to refresh. You can collect the statistics
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immediately or collect them in the background.</p>
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</div>
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<div class="section"><p>There are several scenarios in which the manual management (create,
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remove, refresh, and so on) of column statistics may be beneficial and recommended.</p>
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<dl><dt class="dlterm">High Availability (HA) solutions</dt>
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<dd>When considering the design of high availability solutions where data
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is replicated to a secondary system by using journal entries, it is important
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to know that column statistics information is not journaled. That means that,
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on your backup system, no column statistics are available when
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you first start using that system. To prevent the "warm up" effect that this
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may cause, you may want to propagate the column statistics were gathered on
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your production system and recreate them on your backup system manually.</dd>
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<dt class="dlterm">ISV (Independent Solution Provider) preparation</dt>
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<dd>An ISV may want to deliver a solution to a customer that already includes
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column statistics frequently used in the application instead of waiting for
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the automatic statistics collection to create them. A way to accomplish this
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is to run the application on the development system for some time and examine
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which column statistics were created automatically. You can then generate
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a script file to be shipped as part of the application that should be executed
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on the customer system after the initial data load took place.</dd>
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<dt class="dlterm">Business Intelligence environments</dt>
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<dd>In a large Business Intelligence environment, it is quite common for large
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data load and update operations to occur overnight. As column statistics are
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marked as stale only when they are touched by the Statistics Manager, and
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then refreshed after first touch, you may want to consider refreshing them
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manually after loading the data.<p>You can do this easily by toggling the
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system value QDBFSTCCOL to *NONE and then back to *ALL. This causes all stale
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column statistics to be refreshed and starts collection of any column statistics
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previously requested by the system but not yet available. Since this process
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relies on the access plans stored in the Plan Cache, avoid performing a system
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initial program load (IPL) before toggling QDBFSTCCOL since an IPL clears
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the Plan Cache. </p>
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<p>You should be aware that this procedure works only
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if you do not delete (drop) the tables and recreate them in the process of
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loading your data. When deleting a table, access plans in the Plan Cache that
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refer to this table are deleted. Information about column statistics on that
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table is also lost. The process in this environment is either to add data
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to your tables or to clear the tables instead of deleting them.</p>
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</dd>
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<dt class="dlterm">Massive data updates</dt>
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<dd>Updating rows in a column statistics-enabled table that significantly
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change the cardinality, add new ranges of values, or change the distribution
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of data values can affect the performance for queries when they are first
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run against the new data. This may happen because, on the first run of such
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a query, the optimizer uses stale column statistics to make decisions on the
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access plan. At that point, it starts a request to refresh the column statistics. <p>If
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you know that you are doing this kind of update to your data, you may want
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to toggle the system value QDBFSTCCOL to *NONE and back to *ALL or *SYSTEM.
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This causes an analysis of the Plan Cache. The analysis includes searching
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for column statistics that were used in the generation of an access plan,
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analyzing them for staleness, and requesting updates for the stale statistics. </p>
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<p>If
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you massively update or load data and run queries against these tables at
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the same time, then the automatic collection of column statistics tries to
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refresh every time 15% of the data is changed. This can be redundant processing
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since you are still in the process of updating or loading the data. In this
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case, you may want to block automatic statistics collection for the tables
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in question and deblock it again after the data update or load finishes. An
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alternative is to turn off automatic statistics collection for the whole system
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before updating or loading the data and switching it back on after the updating
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or loading has finished.</p>
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</dd>
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<dt class="dlterm">Backup and recovery</dt>
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<dd>When thinking about backup and recovery strategies, keep in mind that
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creation of column statistics is not journaled. Column statistics that exist
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at the time a save operation occurs are saved as part of the table and restored
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with the table. Any column statistics created after the save took place are
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lost and cannot be recreated by using techniques such as applying journal
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entries. If you have a rather long interval between save operations and rely
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heavily on journaling for restoring your environment to a current state, consider
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keeping track of column statistics that are generated after the latest save
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operation.</dd>
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</dl>
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</div>
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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="statsmanager.htm" title="As stated earlier, the collection of statistics is handled by a separate component called the Statistics Manager. Statistical information can be used by the query optimizer to determine the best access plan for a query. Since the query optimizer bases its choice of access plan on the statistical information found in the table, it is important that this information be current.">Collecting statistics with the Statistics Manager</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../rzakz/rzakzqdbfstccol.htm">Allow background database statistics collection (QDBFSTCCOL) system value</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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