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FIRST_ROWS_n: This optimizing mode uses cost optimization regardless of the availability of statistics. The goal is the fastest response time for the first n number of rows of output, where n can take the value of 10, 100, or 1,000. FIRST_ROWS: The FIRST_ROWS mode uses cost optimization and certain heuristics (rules of thumb), regardless of whether you have statistics or not. You use this option when you want the first few rows to come out quickly so response time can be minimized. Note that the FIRST_ROWS mode is retained for backward compatibility purposes only, with the FIRST_ ROWS_n mode being the latest version of this model.

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You can set the Optimizer mode at the instance, session, or statement level. You set the Optimizer mode at the instance level by setting the initialization parameter OPTIMIZER_MODE to ALL_ROWS, FIRST_ROWS_n, or FIRST_ROWS, as explained in the previous section. For example, you can set the goal of the query optimizer for the entire instance by adding the following line in your initialization parameter file: OPTIMIZER_MODE = ALL_ROWS Setting the initialization parameter OPTIMIZER_MODE to ALL_ROWS ensures that all SQL statements in the instance are optimized. You can also set the Optimizer mode for a single session by using the following ALTER SESSION statement: SQL> ALTER SESSION SET optimizer_goal = first_rows_10; Session altered. SQL> The previous statement directs the Optimizer to base its decisions on the goal of the best response time for getting the first ten rows of the output of every SQL statement that is executed.

Note that the Optimizer mode you choose applies only to SQL statements that are issued directly. If you use an ALTER SESSION statement to change the Optimizer mode for SQL that s part of a PL/SQL code block, it ll be ignored. You must use Optimizer hints, which I discuss in the section titled Using Hints to Influence the Execution Plan, to set the Optimizer mode for any SQL statement that s part of a PL/SQL block.

Another restriction you should be aware of is the lack of const methods. It is important to keep this restriction in mind, because it affects other traditional usages of the keyword const. Even though it is possible to define a variable or a parameter of type const T^, it is seldom useful. Due to the lack of const methods, you would not be able to call any method on such a variable. Without casting away the const modifier, you would only be able to assign a new reference to the variable, or to read a const member variable. However, in managed types, you should not define const data members, as the next section explains.

To determine the current Optimizer mode for your database, you can run the following query: SQL> SELECT name, value FROM V$PARAMETER 2 WHERE name = 'optimizer_mode'; NAME VALUE ----------------------------------optimizer_mode ALL_ROWS SQL> Any SQL statement can override the instance- or session-level settings with the use of Optimizer hints, which are directives to the Optimizer for choosing the optimal access method. By using hints, you can override the instance-wide setting of the OPTIMIZER_MODE initialization parameter. See the section Using Hints to Influence the Execution Plan later in this chapter for an explanation of Optimizer hints.

You get the following output:

The CBO performs several intricate steps to arrive at the optimal execution plan for a user s query. The original SQL statement is most likely transformed, and the CBO evaluates alternative access paths (for example, full-table or index-based scans). If table joins are necessary, the Optimizer evaluates all possible join methods and join orders. The Optimizer evaluates all the possibilities and arrives at the execution plan it deems the cheapest in terms of total cost, which includes both I/O and CPU resource usage cost.