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Theoretically, if your data is static, you may only need to collect statistics once. If your database performs only a small amount of DML activities, you may collect statistics at relatively longer intervals, say weekly or monthly. However, if your database objects go through constant change on a daily basis, you need to schedule the statistics collection jobs much more frequently, say daily or even more often. One of the best Oracle Database 10g new features is that you can let the database decide the frequency of statistics collection. Remember that the database bases its statistics collection on whether the statistics are fresh or stale. Thus, you can relax and let the database be the arbiter of how often to collect statistics.

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You ve seen how the Oracle database can automatically collect Optimizer statistics for you You ve also learned how to use the DBMS_STATS package to collect the statistics manually yourself But what happens if you disable the automatic statistics collection process, or if you don t collect statistics in a timely fashion Even with automatic statistics collection, under which necessary statistics are collected on a nightly basis, you may have a situation where table data is altered after the statistics collection process is over In situations such as this, Oracle uses data, such as the number of blocks taken up by the table data and other ancillary information, to figure out the Optimizer execution plan.

You can also use the initialization parameter OPTIMIZER_DYNAMIC_SAMPLING to let Oracle estimate Optimizer statistics on the fly, when no statistics exist for a table, or when the statistics exist but are too old or otherwise unreliable Of course, sampling statistics dynamically would mean that the compile time for the SQL statement involved would be longer Oracle smartly figures out if the increased compile time is worth it when it encounters objects without statistics If it s worth it, Oracle will sample a portion of the object s data blocks to estimate statistics You need to set the value of the OPTIMIZER_DYNAMIC_SAMPLING initialization parameter to 2 or higher to enable dynamic sampling of all unanalyzed tables Because the default for this parameter is 2, dynamic sampling is turned on by default in your database Thus, you need not spend sleepless nights worrying about objects with missing or outdated statistics.

In any case, if you adhere to Oracle s recommendation and use the Automatic Optimizer Statistics Collection feature, the GATHER_STATS_JOB will automatically collect your database s statistics The GATHER_STATS_JOB is created at database creation time and is managed by the Oracle Scheduler, which runs the job when the maintenance window is opened By default, the maintenance window opens every night from 10 PM to 6 AM, and all day on weekends Oracle will collect statistics for all objects that need them if you adopt the Automatic Optimizer Statistics Collection feature The feature is turned on by default in a new Oracle 10g database or when you upgrade to the 10g release from an older version..

As with so many other DBA tasks in Oracle Database 10g, you re better off simply using the OEM Database Control or the Grid Control to schedule the collection of Optimizer statistics. Here are the steps to collect Optimizer statistics using the Database Control or Grid Control interfaces of the OEM: 1. From the Database Control home page, click the Administration tab. 2. In the Administration page, click the Manage Optimizer Statistics link under the Statistics Management group. 3. You re now in the Manage Optimizer Statistics page. Click the Gather Statistics link to start collecting statistics and follow the instructions for the five steps you must implement. Figure 21-1 shows part of the Optimizer statistics collection process using the OEM Grid Control interface.

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Note Oracle strongly recommends that you just use the Oracle-created GATHER_STATS_JOB, run by the Scheduler during the scheduled maintenance window, to collect Optimizer statistics. You may want to collect Optimizer statistics manually under an extreme situation, such as the database not being up during the scheduled maintenance window, or if you want to analyze a newly created table right away.