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;

Output: user is main_user

If you want to change only the default schema:

ALTER SESSION SET current_schema=other_schema_name