Which views can be queried while instance is up but database not mounted in Oracle 11G?
Generally speaking, most of the dynamic performance views, known as V$ views, which are populated in the memory, SGA to be specific are accessible after the instance startup process. If the these V$ views contains information which are stored in physical files such as control files or data files then your database has to be mounted and/or opened. 

For example, when your database instance is started but database is unmounted, you can describe the v$database view but can't access it's contents until the database is mounted. This is due to the fact that the view has to get information from control files which is inaccessible until the database is mounted.

SQL&gt; startup nomount

Instance is started.

SQL&gt; desc v$database

Can view the structure becuase it is pupulated in SGA.

SQL&gt; select * from v$database;
select * from v$database
              *
ERROR at line 1:
ORA-01507: database not mounted


SQL&gt; alter database mount;

Database altered.

SQL&gt; select * from v$database;

Now, you may access it's contents.


v$instance is accessible after the instance is started. As it's name suggests, it contains information related to memory and processes.