Switching to a Different Schema

The following statement sets the schema of the current session to the schema name specified in the statement.

ALTER SESSION SET CURRENT_SCHEMA = <schema name>;

In subsequent SQL statements, Oracle Database uses this schema name as the schema qualifier when the qualifier is omitted. In addition, the database uses the temporary tablespace of the specified schema for sorts, joins, and storage of temporary database objects. The session retains its original privileges and does not acquire any extra privileges by the preceding ALTER SESSION statement.

You can use this command above to avoid the use of public synonyms.  By setting the CURRENT_SCHEMA attribute to the schema owner name it is not necessary to create public synonyms for production table names. Without the current_schema syntax, you would have to assign a public synonym for every table in your production database.

If you want to check the current schema of the session you can use the following statement:

SELECT sys_context( 'userenv', 'current_schema' ) from dual; 

Let's look in the following example:

CONNECT user1

ALTER SESSION SET CURRENT_SCHEMA = schema_2;

SELECT * FROM empoyers;

Because user1 is not schema-qualified, the table name is resolved under schema schema_2. But if user1 does not have select privilege on table schema_2.empoyers, then user1 cannot execute the SELECT statement even switching the current session schema.