ORA-01034: ORACLE not available

The instance you tried to reach is not running. Either the database is down, or the client is pointing at a different instance than you think. Check the instance and ORACLE_SID, then start the database if it should be running.

What ORA-01034 means

The client reached the machine, but no running instance exists for that name. It often appears together with ORA-27101: shared memory realm does not exist. In a local connection, a wrong ORACLE_SID gives the same result as a stopped database.

Common causes of ORA-01034

The database is stopped, or it crashed.
A wrong ORACLE_SID or ORACLE_HOME in the environment.
The instance failed to start because of a memory or parameter problem.
On Windows, the OracleService for that SID is stopped.

How to fix ORA-01034

On the server, check for background processes (ps -ef | grep pmon) or the Windows service.
Verify ORACLE_SID and ORACLE_HOME.
Read the alert log to find out why the instance stopped.
Start it: STARTUP as SYSDBA. In a multitenant database, also check that the PDB is open.

ORA-01034 example

On the database server:

ps -ef | grep pmon
echo $ORACLE_SID

sqlplus / as sysdba
SQL> STARTUP
SQL> SELECT name, open_mode FROM v$pdbs;

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Oracle documentation for ORA-01034. Oracle is a registered trademark of Oracle Corporation. SQLlightspeed is not affiliated with Oracle.