Instance Status RESTRICTED in Oracle Listener

When you don't want any user to connect to database, you can simply restrict your instance from any incoming connection. But if you don't know this concept and you have come across the below error, it means your database is in sort of maintenance mode and you need to disable it to enable users to connect to it via listener.

Note: Maintenance mode still allows users to connect to database without listener, of course you can only do it when you are on DB server and you are not using listener for DB connection i.e. no use of @SID.

ORA-12154: TNS: could not resolve the connect identifier specified

When listener is not aware of your database, you might come across the below error when trying to connect to a database.

PS C:\Users\parvi> sqlplus sys@practice as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Tue Jan 7 21:16:26 2025
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.

Enter password:
ERROR:
ORA-12154: TNS:could not resolve the connect identifier specified

You'll need to make your instance known to listener in this case, there are multiple ways to do this, but I'll specify the 2 easiest ones which will help you fix this error.

libnjni12.so(), but a runtime definition of the symbol was not found

/oracle/app/oracle/product/12.1.0/dbhome_1/lib/libnjni12.so(), but a runtime definition of the symbol was not found.

libnjni12.so(), but a runtime definition of the symbol was not found

UNNAMED Datafile | ORA-01111: name for data file is unknown | Oracle Standby

ORA-01111 ORA-01110 ORA-01157 is Oracle Standby Alert Log | Datafile created with UNNAME#### in Oracle Standby after adding new datafile to Primary

UNNAME#### in Oracle Standby after adding new datafile to Primary

ORA-01113: file # needs media recovery | Oracle Database

ORA-01113: file # needs media recovery | ORA-01110: data file #: 'E:\Database\TESTDB\#####.DBF'

ORA-01113 Oracle error indicates inconsistent datafiles with respect to Controlfile SCN and hence requiring further recovery in order to be online again. How those datafiles got into this RECOVER state can be because of multiple reasons as we can assume the different possibilities (power failure, server crash etc). But how they can be recovered and brought back online is answerable and simple.

Unable to connect to PDB | ORA-12154 ORA-12518 | Oracle 12c

ORA-12154: TNS:could not resolve the connect identifier specified | ORA-12518: TNS:listener could not hand off client connection

ORA-12154 and ORA-12154 generating when you are having issues with database connectivity whenever you try to login into your database. In my scenario, I was trying to install and configure Oracle Enterprise Manager 12c on my local system for pratice purposes and for that I had installed repository database named as EMCTLDB. This is a 12c database where EMCTLDB is the CDB name and EMCTLPDB is the PDB name. After the database was installed, I made the entries in the LISTENER.ORA and TNSNAMES.ORA like this,