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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.
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Note 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.
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I discuss the Oracle optimizer in detail in 19, in the context of performance tuning.
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You can also use the Oracle optimizer in an enhanced tuning mode, as shown in 19. The Oracle optimizer in the tuning mode is the basis of the SQL Tuning Advisor feature, also explained in 19.
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Talking to the Database
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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, SQL Developer, and the OEM Database Control and Grid Control interfaces, which serve as the main DBA management consoles.
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Connecting to Oracle
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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.
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CHAPTER 5 ORAC LE DATABA SE 1 1G AR CHITECTURE
Note
Oracle Net Services is Oracle s mechanism for interfacing with the communication protocols (TCP/IP for example) that define the way data is transmitted on a network.
Since a connection to a database and a user session are closely related to a user process but are actually quite different from each other, let me take a minute to explain the differences between a connection and a session. An Oracle connection represents a communication link between a user s process and an Oracle Database instance. This communication link, or pathway, can be located on the same server as both the user process and the database server process run. The communication pathway can also be established using network software, as is the case when the client runs on one computer and the Oracle database on another, with both of them communicating via the network. A session represents a specific user connection to a running Oracle instance via a user process. A session starts when, for example, you start SQL*Plus and log into the database, and it lasts until you disconnect or exit. You can create multiple simultaneous sessions under the same Oracle username and password credentials. Under the common dedicated server approach, as you saw earlier, the database creates a dedicated server process to serve each user session. When you use a shared server approach, multiple user sessions share a single server process. For a connection to succeed, the client application must specify the location of the database. On the database side, the Oracle Net listener, known simply as the listener, is the process that listens for incoming client connection requests. You configure the listener in the listener.ora file, where you provide the database address. The listener.ora file also defines the protocol the listener is listening on and related information. On the client side, you can either use the tnsnames.ora file to list the database server connection details, which include the database name, server name, and connection protocol, or use the much simpler easy connect method.
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