Today we will discuss the third topic in oracle which is called Selecting an Instance with Environment variables.In this topic we will learn about various kinds of instance and there variables.How they work and in which environment they execute properly.


Selecting an Instance with Environment Variables

Before you attempt to use SQL*Plus to connect locally to an Oracle instance, you must ensure that environment variables are set properly. When multiple database instances exist on one server, or when an Automatic Storage Management (ASM) instance exists on the same server as one or more database instances, the environment variables determine which instance SQL*Plus connects to. (This is also true when there is only one Oracle instance on a server.)
For example, each Oracle instance (database or ASM) has a unique system identifier (SID). To connect to an instance, you must at a minimum set the ORACLE_SID environment variable to the SID of that instance. Depending on the operating system, you may need to set other environment variables to properly change from one instance to another.
Refer to the Oracle Database Installation Guide or administration guide for your operating system for details on environment variables and for information on switching instances.
Note:This discussion applies only when you make a local connection—that is, when you initiate a SQL*Plus connection from the same machine on which the target instance resides, without specifying an Oracle Net Services connect identifier. When you make a connection through Oracle Net Services, either with SQL*Plus on the local or a remote machine, or with Enterprise Manager, the environment is automatically set for you.
For more information on connect identifiers, see Oracle Database Net Services Administrator's Guide.
Solaris Example
The following Solaris example sets the environment variables that are required for selecting an instance. When switching between instances with different Oracle homes, the ORACLE_HOME environment variable must be changed.% setenv ORACLE_SID SAL1
% setenv ORACLE_HOME /u01/app/oracle/product/10.1.0/db_1
% setenv LD_LIBRARY_PATH /usr/lib:/usr/dt/lib:/usr/openwin/lib:/usr/ccs/lib
Most UNIX installations come with two scripts, oraenv and coraenv, that can be used to easily set these environment variables. For more information, see Administrator's Reference for UNIX Systems.
Windows Example
On Windows, you must set only the ORACLE_SID environment variable to select an instance before starting SQL*Plus.SET ORACLE_SID=SAL1


This concludes our topic i hope you have read it in detail and understand the concept of the topic.Our next topic is Identifying Your Oracle Database Software Release.Which i will post soon