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SQL*Net Dedicated Process

Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

The Internals Of Oracle's SQL*Net

1. Issue a remote request. Check the database link called LONDON.


2. Database link. Get the service name (london_unix_d) using the link_name (LONDON).

CONNECT TO london_unix_d;

3. tnsnames.ora. Get the sid name (london_sid) using service name (london_unix_d).

london_unix_d = (description=(address=(protocol=tcp) (host=seagull)
(port=1521) (connect_data=(sid=london_sid) (server=dedicated)))

4. etc/hosts. Get the IP address ( using the sid name (london_sid). london_sid london_unix.corporate.com

As you can see, this translation occurs in a multistage process. The tnsnames.ora file specifies the name of the host containing the destination database. For Unix environments, the host name is then looked up in the etc/hosts file to get the IP address of the destination box.

The service name is looked up in tnsnames.ora. If the service exists, the IP address is found in the etc/hosts file and a communications request is sent to the destination IP address. Note that both of the entries in this file connect to London, but london_unix_d directs SQL*Net to spawn a dedicated process, while london_unix uses the multithreaded server component because a shared server is specified.

Now that you have the tnsnames.ora and etc/hosts files in place, you can include any tables from the London sites by qualifying the remote site name in the SQL query. For example:

SELECT CUSTOMER.customer_name, ORDER.order_date
FROM customer@london, order

This query joins two tables at different locations, and the database link called london determines how the Oracle connection will be established on the destination system. Regardless of how the connection is made to the destination, however, the user ID must have SELECT privileges against the customer table, or this query will fail.

This is an excerpt from "High Performance Data Warehousing", copyright 1997.
If you like Oracle tuning, you may enjoy the book Oracle Tuning: The Definitive Reference , with over 900 pages of BC's favorite tuning tips & scripts. 

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