====== Connect as another user (using a privileged one)  ======

<code sql>
create user proxy1 identified by proxy1;
alter user main_user grant connect through proxy1;
connect proxy1[main_user]/proxy1@SID;
</code>

Now we are connected, let's check current user:

<code sql>
show user;
</code>

Output: 
user is main_user

====== Change only the default schema ======

<code sql>
alter session set current_schema=other_schema_name
</code>

Check if the schema was changed:

<code sql>
select sys_context( 'userenv', 'current_schema' ) from dual;
</code>