set serveroutput on;
declare
l_query clob;
l_count number;
l_user varchar2(200);
begin

select user into l_user from dual;
dbms_output.put_line(l_user);
execute immediate 'ALTER SESSION SET CURRENT_SCHEMA =APPS';

select user into l_user from dual;
dbms_output.put_line(l_user);
--l_query:='select count(*) from CDR_W520_2897CC0E.T681036830';
--execute immediate l_query into l_count;
dbms_output.put_line('count is='||l_count);
EXCEPTION 
  when others 
    then dbms_output.put_line('EXCEPTION is='||l_count);
end;
/
show errors



Output:
--------
anonymous block completed

MySchema
MySchema
count is=

No Errors.

execute immediate 'ALTER SESSION SET CURRENT_SCHEMA =APPS';

rclz>
orclz> conn scott/tiger
Connected.
orclz> select sys_context('userenv','current_schema') from dual;

SYS_CONTEXT('USERENV','CURRENT_SCHEMA')
--------------------------------------------------------------------------------------------------------------
SCOTT

orclz> alter session set current_schema=system;

Session altered.

orclz> select sys_context('userenv','current_schema') from dual;

SYS_CONTEXT('USERENV','CURRENT_SCHEMA')
--------------------------------------------------------------------------------------------------------------
SYSTEM

orclz> select sys_context('userenv','current_user') from dual;

SYS_CONTEXT('USERENV','CURRENT_USER')
--------------------------------------------------------------------------------------------------------------
SCOTT

orclz>