Connect to database with SQL*Plus: is TNSNAMES.ORA used?

Question

Hi ,
we have a local database called DEV01 and a duplicate of this database in a datacenter. To be able to connect to both databases the TNSNAMES.ORA
on the local database server (not the one in the datacenter) has the following entries:
/* Entry for local database */
DEV01_local, DEV01_local.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = server01)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SID= DEV01)
/* Entry for database in datacenter */
DEV01,DEV01.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = server02)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SID = DEV01)
When I connect to the local database server I set the Oracle-environment as follows:
export ORACLE_SID=DEV01
export PATH=/opt/oracle/app/oracle/product/9.2.0/bin:$PATH
export ORACLE_HOME=/opt/oracle/app/oracle/product/9.2.0
Now I start SQL*Plus as follows:
oracle#> sqlplus user/user
SQL> select instance_name, host_name from v$instance;
INSTANCE_NAME    HOST_NAME
DEV01            server01
Since the SID is on both databases the same (DEV01) I really do not understand how Oracle decides to which database SQL*Plus connects to.
Is it a matter of the entries in the TNSNAMES.ORA?
But when I start SQL*Plus as follows:
oracle#> sqlplus user/user@DEV01_local
SQL> select instance_name, host_name from v$instance;
INSTANCE_NAME    HOST_NAME
DEV01            server01
or
oracle#> sqlplus user/user@DEV01
SQL> select instance_name, host_name from v$instance;
INSTANCE_NAME    HOST_NAME
DEV01            server02
everything is as expected.
Does anybody know which database gets chosen when just connect with "sqlplus user/user" (without @DEV01 or @DEV01_local)?
Any help will be appriciated!
Rgds
Jan

Answer 1

Both databases have the SID=DEV01.
The ORACLE_SID is set to DEV01 - but when connecting I end up at
oracle#> sqlplus user/user
SQL> select instance_name, host_name from v$instance;
INSTANCE_NAME    HOST_NAME
DEV01            server01
which is the database with the TNSNAMES.ORA entry "DEV01_local, DEV01_local.WORLD = ..."
In your case the one defined by ORACLE_SID environment variable => but which from both is it?