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In addition to the data dictionary, Oracle maintains an important set of dynamic performance tables. These tables maintain information about the current instance, and Oracle continuously updates these tables. The set of virtual dynamic tables is referred to as the X$ tables. Oracle doesn t allow you to access the X$ tables directly; rather, Oracle creates views on all these tables and then creates synonyms for these views. You ll be accessing these views, called the V$ views, to get information about various aspects of a running instance. The V$ views are the foundation of all Oracle database performance tuning. If you wish to master the Oracle database, you must master the V$ dynamic views, because they are the wellspring of so much knowledge about the Oracle instance. The dynamic performance views, like the data dictionary views, are based on read-only tables that only Oracle can update. Some of the tables capture session-wide information, and some of them capture system-wide information. You ll find the dynamic views extremely useful in session management, backup operations and, most important, in performance tuning. Remember, though, that the dynamic performance tables are only populated for the duration of the instance and are cleaned out when you shut down the instance.

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In most cases, when users issue a query against the database, there s more than one way to access the tables and retrieve the data. Because there are many ways to execute the same statement, Oracle uses a cost-based optimizer (CBO) to choose the best execution plan for queries, based on the cost of the query in terms of resource use. Query optimizing is at the heart of modern relational databases and is an essential part of how Oracle conducts its operations. The query optimizer is transparent to users and Oracle will automatically apply the best access and join methods to your queries before it starts processing.

Since all the primitives shown here, and their native equivalents, have the same binary layout, a primitive type in managed code can still be used as a native primitive, if this is required by the context. The following sample code shows this. Even though a variable of type System::Double is passed to std::cout, the code compiles and works as expected. // primitives.cpp // build with "CL /clr primitives.cpp" #include <iostream> using namespace std; int main() { System::Double PI = 3.14159265358979; cout << PI; }

To choose the best execution plans, Oracle uses statistics on tables and indexes, which include counts of the number of rows and the data distribution of data skew in the tables within the database. (The physical storage statistics and the data distribution statistics for all database tables and indexes, columns, and partitions are stored in various data dictionary tables.) Armed with this information, the optimizer usually succeeds in finding the best path to access the necessary data for executing a SQL statement. Oracle also lets you use hints to override the optimizer s choice of an execution path. This is because in some instances the application developer s knowledge of the data enables the use of more efficient execution plans than the optimizer can come up.

In Oracle Database 10g, you can also use the Oracle optimizer in an enhanced tuning mode, as shown in 21. The Oracle optimizer in the tuning mode is the basis of the new SQL Tuning Advisor feature, also explained in 21.

There is also a Perl function for performing the same task. You can access it like this:

In order for a user to communicate with the database, he or she must first connect to the database by creating a user session. The user communication with the database is done through one of several interfaces. This section will quickly review Oracle database connectivity aspects and the main communication interfaces, including SQL*Plus, iSQL*Plus, and the OEM Database Control and Grid Control interfaces, which serve as the main DBA management consoles.

You can connect to the Oracle database from the server on which the Oracle RDBMS is running. However, DBAs as well as application developers and users generally connect to the database through the network using Oracle Net, a component of Oracle Net Services. Oracle Net enables network sessions from a client application to an Oracle database server. It acts as the data courier for the clients and the database server, and it is in charge of establishing and maintaining the connection as well as transmitting messages between client and server. Oracle Net is installed on each computer in the network.

Oracle Net Services is Oracle s mechanism for interfacing with the communication protocols (TCP/IP, FTP, and so on) that define the way data is transmitted and received on a network.