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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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The ATO makes sure that there are representative, up-to-date statistics for all the objects in the SQL statement, which you need for efficient execution plans. If the ATO finds any statistics that are missing or stale, it suggests that you collect new statistics for the objects in question. During this process, the ATO collects other information that it can use to fill in any missing statistics. It can also correct stale statistics.
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At this stage the ATO tries to verify the validity of its estimates of factors such as column selectivity and cardinality of database objects. It can use three methods to verify its estimates: Dynamic data sampling: The ATO can use a data sample to check its estimates. The ATO can apply correction factors if the data-sampling process shows its estimates to be significantly wrong. Partial execution: The ATO can carry out the partial execution of a SQL statement. This process allows it to check whether its estimates are close to what really happens. It does not check whether its estimates are correct, but rather it checks whether a plan derived from those statistics is the best possible plan. Past execution history statistics: The ATO can use the SQL statement s execution history to help with its work. If there s enough information from statistics analysis or SQL profiling, the ATO suggests you create a SQL profile, which is supplementary information about a SQL statement. If you accept this advice and are running the optimizer in tuning mode, Oracle will store the SQL profile in the data dictionary. Once you have done this, the optimizer uses it to produce optimal execution plans, even when it is running in normal mode.
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Remember that a SQL profile is not the same thing as a stored execution plan.
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The SQL profile will continue to apply if you make small changes to your database and allow your objects to grow normally. One of the big advantages of SQL profiles is the ability to tune packaged applications. These are hard to tune because you can t easily access and modify the code. Because SQL profiles are saved in the data dictionary, you can use them to tune packaged applications.
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The ATO analyzes how using an improved access method, such as working with an index, will affect queries. These are important considerations, because adding an index can substantially increase the speed of a query. However, adding new indexes can adversely affect other SQL statements; the SQL Advisor knows this and makes its recommendations as follows: If an index is effective, it will advise you to create it. It can advise you to run the SQL Access Advisor (see 7 for details) to analyze the wisdom of adding the new index.
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CHAPTER 19 IM PR OVING DA TA BAS E PERFORM ANC E: S QL QUE RY OPTIMIZA TION
SQL Structure Analysis
The ATO can make recommendations to modify the structure (both the syntax and semantics) of poorly performing SQL statements. The ATO considers issues such as the following: Design mistakes; for example, performing full table scans because you didn t create indexes. Using inefficient SQL; for example, the NOT IN construct, which is known to be much slower than the NOT EXISTS construct in general.
Note
The ATO only identifies poorly written SQL, but it won t rewrite it for you. You will know your application better than the ATO, so Oracle only provides advice, which you can implement or not.
Recommendations
Here are some recommendations that the SQL Tuning Advisor will give you: Creating indexes will speed up access paths. Using SQL profiles will allow you to generate a better execution plan. Gathering optimizer statistics for objects that do not have any, or renewing stale statistics, will be of benefit. Rewriting SQL as advised will improve its performance.
The SQL Tuning Advisor in Practice
You can use the SQL Tuning Advisor through packages or through the web interface of the OEM Database Control.
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