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The CBO can follow optimal execution paths only if it has detailed knowledge of the database objects. In Oracle Database 10g, the recommended way to provide these statistics is by letting the database automatically collect statistics for you. This is known as the Automatic Optimizer Statistics Collection feature, which I explained in 17. You can also manually provide statistics to the Optimizer with the DBMS_STATS package. Note that whether you rely on automatic collection of statistics or collect them yourself manually, Oracle uses the DBMS_STATS package to collect statistics.

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Although letting the database automatically collect Optimizer statistics is the recommended approach under Oracle Database 10g, you can still manually collect Optimizer statistics using the DBMS_STATS package.

For large tables, Oracle recommends just sampling the data, rather than looking at all of it. Oracle lets you specify row or block sampling, and it sometimes seems to recommend sampling sizes as low as 5 percent. The default sampling size for an estimate is low too. Oracle also recommends using the DBMS_STATS automatic sampling procedure. However, statistics gathered with sampled data aren t reliable. The difference between collecting Optimizer statistics with the estimate at 30 percent and 50 percent is startling at times in terms of performance. Always choose the option of collecting full statistics for all your objects, even if the frequency is not as high as it could be if you just sampled the data.

The following example searches for space characters within each line of input and replaces them with the string %20. The search-and-replace syntax follows the pattern s/search string/replacement string/. The g at the end of the expression is optional; it stands for global and indicates that you want to replace all instances of the search term found in the line. Without the g, the command replaces only the first instance of the search term.

As I explained in 17, you must manually collect Optimizer statistics under the following conditions: When you use external tables When you need to collect system statistics To collect statistics on fixed objects, such as the dynamic performance tables (for dynamic tables, you should use the GATHER_FIXED_OBJECTS_STATS procedure to collect Optimizer statistics) Immediately after you run a bulk load job, because this makes your automatically collected statistics unrepresentative The following sections show you how to make use of the DBMS_STATS package to gather statistics.

Note Oracle recommends that you not use the older ANALYZE statement to collect statistics for the Optimizer, but rather use the DBMS_STATS package. The ANALYZE command is retained for backward compatibility, and you must use it for non-Optimizer statistics collection tasks, such as verifying the validity of an object (using the VALIDATE clause), or identifying migrated and chained rows in a table (using the LIST CHAINED ROWS clause).

Even though C++/CLI allows you to define static and non-static const fields, I recommend using initonly fields instead of const fields, and static initonly fields or literal fields instead of static const fields. Since the CTS does not have natural support for const fields, C++/CLI uses special extensibility features of the CTS (so-called optional signature modifiers) if you define a const field. These signature modifiers are ignored by other .NET languages. Furthermore, they are not used during JIT compilation to optimize the generated code or to verify type safety of IL code. When implementing managed types, the only scenario in which const still makes sense is the definition of const local variables of primitive types. The following code shows an example: void R::f() { const double PISquare = Math::PI * Math::PI; // ... do something with PISquare here }

You use various DBMS_STATS package procedures to collect Optimizer statistics. Most of these procedures have three common attributes STATOWN, STATTAB, and STATID which enable you to save the collected statistics in a database table owned by a user. By default, these attributes are null, and you shouldn t provide a value for any of these attributes if your goal is to collect statistics for the Optimizer. When you ignore these attributes, Optimizer statistics you collect are stored in the data dictionary tables by default, where they re accessible to the Oracle Optimizer.